Clarifications for third party library use in the contribution checklist.

Docs/Reference.dox

@@ -3232,9 +3232,17 @@ Third, there are requirements for new code that come from Urho3D striving to be
 
 - Mark all application-usable classes, structs and functions with the macro URHO3D_API so that they are exported correctly in a shared library build.
 
+- 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.
+
+\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.
 
-- 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.
+- Prefer small and well-focused libraries for the Urho3D runtime. For example we use stb_image instead of FreeImage to load images, as it's assumed that the application developer can control the data and do offline conversion to supported formats as necessary.
+
+- For libraries that would be mandatorily part of the Urho3D build (not switchable off via CMake options), use of C++11 features is not yet acceptable. The same applies to your own code.
+
+- Third-party libraries should not leak C++ exceptions or use of STL containers into the Urho3D public API. Do a wrapping for example on the subsystem level if necessary.
 */
 
 }