Update README.md to include compilation error fix

A PR and issue were already submitted related to this but they are dead
in the water for other reasons. I needed this fix as well, so I assume this isn't a one-off
error and that this information may be helpful to others.

README.md

@@ -31,6 +31,7 @@ resulting libraries will be placed under `bin/` and the generated headers will b
 
 > Include `use_llvm=yes` for using clang++
 
+> You may need to specify `headers=../godot_headers` if you have compilation issues related to missing include files
 
 And our directory structure will be
 ```