Add note of documentation to the contribution chceklist.

Docs/Reference.dox

@@ -3242,6 +3242,8 @@ Third, there are requirements for new code that come from Urho3D striving to be
 
 - Please heed all the \ref CodingConventions "coding conventions" and study existing code where unsure. Ensure that your text editor or IDE is configured to show whitespace, so that you don't accidentally mix spaces and tabs.
 
+- Whenever you are creating a major new feature, its usage should be documented in the .dox pages in addition to the function documentation in the header files. Create a new page if necessary, and link from an appropriate place in the existing documentation. If it's for example a new rendering feature, it could be linked from the \ref Rendering page. If the feature introduces a completely new subsystem, it should be linked from the main page list in Urho3D.dox.
+
 \section ContributionThirdParty Third party library considerations
 
 - When you add a new third-party library, insert its license statement to the License.txt in the root directory. Only libraries with permissive licenses such as BSD/MIT/zlib are accepted, because complying for example with the LGPL is difficult on mobile platforms, and would leave the application in a legal grey area.