Update README.md to include compilation error fix

Kowbell · Kowbell · commit 2bed3eeb49b8 · 2018-06-19T20:06:35.000-04:00
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.
diff --git a/README.md b/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
 ```
