doc: add MAINTAINING.md

- This document is the result of original work by @haukepetersen and
  refinement by @miri64, @aabadie, @kaspar030, @jcarrano,
  @PeterKietzmann, @ZetaR60, and @jia200x.

MAINTAINING.md

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+# RIOT Maintainer Guidelines
+
+This list presents a series of guidelines for maintainers, including technical
+and non-technical. The list is not exhaustive, and represents a baseline on
+things that should be ensured for contributions.
+
+Notes:
+-   Any of the items in this list might be skipped, if clearly and logically
+    articulated reasons are presented.
+-   The order of the steps is meant to maximize efficiency and minimize
+    overhead, with the philosophy that failing in step n makes steps n+x
+    obsolete. However, this efficiency can depend on the quality of the
+    submission itself. If the PR (Pull Request) is clearly not in a reviewable
+    state (for example, due to poor code cleanliness or overall design), it
+    might be more efficient to give the contributor some broader comments for
+    improvement before further review.
+-   This is a working document and any changes, additions, or other discussions
+    are encouraged, via PRs raised against the document.
+
+The [list of maintainers] gives information on the current maintainers and the
+areas of RIOT they each maintain.
+
+
+## Technical guidelines
+
+### 1. - Review the fundamentals
+
+Before spending the time on an in-depth code review, it's important to assess
+the overall validity of the PR.
+
+1.  Does the reasoning for this PR make sense? \
+    Are the requirements and design goals clearly thought out and clearly
+    expressed? \
+    Is the problem that the PR intends to solve clearly stated?
+2.  Is the solution presented in the PR as simple as possible to satisfy the
+    requirements, but no simpler?
+3.  Is the solution well designed on a high level?
+4.  Do the concepts used by the PR make sense?
+5.  Does the PR break with existing concepts?
+6.  Is the structure of the PR itself valid?
+7.  Are there clear and adequate instructions on how to test the PR? \
+    This may or may not include implemented tests as part of the PR.
+8.  Does the code compile and run?
+9.  Does this PR respect the rights of previous authors, either through
+    retaining their commits or by retaining their copyrights in the boilerplate
+    headers?
+10. Is the PR a duplicate of another PR?
+
+
+### 2. - Review the design of the code
+
+The following list is not exhaustive and addresses the coding issues we have
+regularly seen in the past. In particular, check that the [Best Practices]
+are followed. These checks can be aided (but not replaced) by a tool such as
+Coccinelle, using the script found in dist/tools/coccinelle.
+
+1.  Check for code duplication
+2.  [Check memory usage][Comparing build sizes]
+3.  Check all code paths
+4.  Check for API compliance
+5.  Check for consistent error handling
+6.  Check scope of variables and functions
+7.  Check for syntactical, semantical or logical errors
+8.  Check for any runtime efficiency improvements that can be made in specific
+    lines of code - i.e., without changing the overall design of the code
+9.  Check that the code design is as simple as possible to solve the problem,
+    but no simpler
+
+
+### 3. - Test the PR
+
+Run tests to verify the correct behavior (see 1.6), both on `native` and on a
+few selected boards, or present clearly and logically articulated reasons for
+skipping some/all tests.
+
+
+### 4. - Review the code against the coding conventions
+
+Check that the code follows the [Coding Conventions]. This can be aided (but not
+replaced) by Uncrustify, using the uncrustify-riot.cfg file found in the base
+directory. Note the difference between personal coding style, which is
+subjective and is allowed subject to the other guidelines, and the coding
+conventions, which are absolute and must always be followed.
+
+
+### 5. - Review the documentation
+
+The aim of the documentation is to ensure that the code can be picked up as
+easily as possible in the future. Ideally, the documentation is sufficiently
+complete that no input from the original developer or maintainer is required.
+
+1.  Check for sufficient high-level (module-level) documentation
+2.  Verify function level documentation
+3.  Are critical/hard to understand parts in the code documented?
+4.  Check grammar and spelling of documentation
+
+
+## Non-technical guidelines
+
+### Interaction with contributors
+
+-   Be responsive. Even if you are too busy to review the contribution, try to
+    add a note fairly soon after the PR is submitted, thanking them for their
+    awesome contribution and saying that you will review it in due course. Once
+    the contributor has made their changes, ensure you reply to them in a
+    reasonable timeframe. Acknowledge their replies to concerns if you're happy
+    with their argument.
+-   Be helpful. Give precise and correct advice when possible and when it will
+    help the contributor. This can include code snippets, links to
+    code/issues/wiki entries/web pages, or anything else. Educating contributors
+    means we are investing in our community.
+-   Be friendly. Be objective in your review, and of course, follow the
+    [Code of Conduct].
+
+
+### Organisation of reviewing between maintainers
+
+This section is a placeholder for further discussion around making the review
+process more efficient and effective when multiple maintainers are involved.
+This could include:
+
+-   Usage of labels
+-   Division of review responsibilities, e.g. with ACKs for different areas of
+    review
+-   Usage of GitHub functionality, for example "Reviewers" and "Assignees" lists
+
+[list of maintainers]: https://github.com/RIOT-OS/RIOT/wiki/Maintainers
+[Best Practices]: https://github.com/RIOT-OS/RIOT/wiki/Best-Practice-for-RIOT-Programming
+[Comparing build sizes]: https://github.com/RIOT-OS/RIOT/wiki/Comparing-build-sizes
+[Coding Conventions]: https://github.com/RIOT-OS/RIOT/wiki/Coding-conventions
+[Code of Conduct]: https://github.com/RIOT-OS/RIOT/blob/master/CODE_OF_CONDUCT.md