message: Update README.md
  Add limitation line regarding GCC O2 and O3 not supported.
diff: @@ -156,6 +156,7 @@ Open the tests/all_tests.vcxproj project file with Visual Studio. Build and run
  ## Limitations
  * Currently only GCC, Clang and MSC++ are supported.
  +* On GCC, optimization flag O2 and O3 are not supported. You bust compile the test project with -O1 or -O0.
  * In MSC++, your project must have Edit And Continue debug mode on (https://msdn.microsoft.com/en-us/library/esaeyddf.aspx) which is same of /ZI compiler switch. If you don't use this, you will have exceptions mocking destructors (which includes unique_ptr and other smart pointers).
  * Can't mock classes with multiple inheritance.
  * Can't mock classes with virtual inheritance.
  