Added comment on CMake FindBullet requiring recent versions of CMake.

Modules/Contents/CMakeLists.txt

@@ -3,6 +3,8 @@ IF(BOX2D_FOUND)
     ADD_SUBDIRECTORY(2DPhysics)
 ENDIF(BOX2D_FOUND)
 
+# Note that FindBullet requires CMake >= 2.8.4 to find Bullet >= 2.77 
+# due to Bullet's header install directory location changing.
 FIND_PACKAGE(Bullet)
 IF(BULLET_FOUND)
     ADD_SUBDIRECTORY(3DPhysics)