Adding a porting note for build system.

[ci only: Web] [skip appveyor]

Docs/Urho3D.dox

@@ -1071,6 +1071,7 @@ From 1.6 to master:
 - UIElement::LoadChildXML() now returns the created element on success, instead of a boolean.
 - Rendertargets have gained the ability to have automatically regenerated mipmaps. Screen buffers received from the Renderer subsystem have mipmaps off, but for manually created rendertargets the default is mipmaps on, like for ordinary textures. Use SetNumLevels(1) to disable.
 - The shader function GetWorldTangent() now returns a 4-component vector.
+- Build system - the "Urho3D-CMake-common.cmake" file is now renamed to "UrhoCommon.cmake".
 */
 
 }