buildsystem: document EXTERNAL_PKG_DIRS feature

pkg/doc.txt

@@ -72,5 +72,34 @@
  *  * commit your changes using `git commit`
  *  * create the patch files using `git format-patch -n riot-port`
  *  * move the resulting patch files to the patches directory of your package.
+ *
+ * Packages outside of RIOTPKG
+ * ---------------------------
+ * It can be beneficial to create packages outside of the RIOT tree. For example
+ * if one is working on new packages that aren't ready to be committed to
+ * upstream or if an application needs its own unique packages. For this, one
+ * can use the `EXTERNAL_PKG_DIRS` make variable. It works similar to the way
+ * [external modules](src/creating-modules.html#modules-outside-of-riotbase) are
+ * handled. In your application's Makefile, in addition to adding the package
+ * name to `USEPKG` as shown above, add the path to a folder that contains your
+ * external packages:
+ *
+ * ~~~~~~~~ {.mk}
+ * EXTERNAL_PKG_DIRS += <PATH_TO_FOLDER_CONTAINING_PACKAGES>
+ * ~~~~~~~~
+ *
+ * The path is allowed to be relative to the application's directory.
+ *
+ * ***NOTE:*** The name of an external package must be unique (both in regard to
+ * other external packages, as well to native RIOT packages). Additionally, the
+ * directory containing the packages must match the package name, e.g. package
+ * `foo`must be located in `<PATH_IN_EXTERNAL_PKG_DIRS>/foo`.
+ *
+ * An example can be found in
+ * [`tests/external_pkg_dirs`](https://github.com/RIOT-OS/RIOT/tree/master/tests/external_pkg_dirs)
+ *
+ * RIOT welcomes new and useful packages. If you'd like to share your work,
+ * consider [contributing](https://github.com/RIOT-OS/RIOT/tree/master/CONTRIBUTING.md).
+ *
  * @}
  */