CONTRIBUTING: replace 'help wanted' by 'contributing code'

The naming of the 'help wanted' section is unclear, and unhelpful.
Additionally, most of the items listed in it are not actually part of
the contribution process:

- While *Documentation* is welcome, it is not helpful to document the
  kinds in the contributing document. The contributing document should
  document *how*, not *what*.

- The *Issues* section is out of place: I doubt anyone will look at the
  *contributing guidelines* to find out how to open an issue.
  The content is also pretty self-explanatory.

- What's left is only 'Contributing Code', so promote it!

CONTRIBUTING.md

@@ -5,7 +5,7 @@ contribute, and we appreciate all of them. You can jump to the major sections
 of this document using the following links:
 
 * [Getting Started][getting-started]
-* [Help wanted][help-wanted]
+* [Contributing code][contributing-code]
 * [General Tips][general-tips]
 * [Feature Requests][feature-requests]
 * [Bug Reports][bug-reports]
@@ -54,47 +54,8 @@ contribution into RIOT master faster:
   Alternatively comprehensive testing procedures should be provided with your
   pull request.
 
-## Help Wanted
-[help-wanted]: #help-wanted
-In case you're not really sure where to start, we've created a list of suggestions.
-
-### Documentation
-If you've found yourself struggling to understand a particular aspect, chances
-are you're not the first and won't be the last. Writing down what you've learned
-is a great way to recap your new knowledge and share it with others. You can
-also start to learn about RIOT by combing through existing documentation and
-fixing errors and typos. Any help with improving the documentation is greatly
-appreciated and makes a big difference to the RIOT project.
-After you've finished writing, please publish your documentation in one of the
-following ways, depending on its type.
-
-#### General knowledge, HOWTOs
-Articles that focus on design aspects or how to use a particular module should
-be contributed to the [RIOT wiki](https://github.com/RIOT-OS/RIOT/wiki). After
-you've added your entry, please share it on the riot-dev mailing list so
-everyone is aware of its existence (and thank you).
-If you'd like to document a solution to minor annoyances or common pitfalls,
-please do not hesitate to extend the [Troubleshooting wiki
-page](https://github.com/RIOT-OS/RIOT/wiki/Troubleshooting). Again, please share
-your additions with the riot-dev mailing list.
-
-#### Code comments, HOWTOs for particular projects
-Documentation that relates directly to the code at hand like the HOWTO files
-that can be found in some of the directories in ``RIOT/examples/`` or comments
-in the code itself should be submitted through a [pull request][pull-requests].
-
-If you're not sure about the correct way to submit your writing, please ask on
-the mailing list or open an issue saying which documentation is missing. The
-other RIOTers will help you find the right format.
-
-### Issues
-If RIOT behaves oddly, please do not hesitate to [open an issue][open-an-issue].
-Other RIOT developers will be happy to help figure out what the problem is and
-fix possible bugs. Please notice that we use a bunch of tags to label the
-issues. If you have permission to use them, do it. Their meanings are explained
-[here][labels].
-
-### Contribute code
+## Contributing code
+[contributing-code]: #contributing-code
 If you think your work should be integrated in the main RIOT repository, take
 the following steps: (short version, the more detailed version can be found
 [below][pull-requests])