Update README for reflection source
Generated by flatcc
Keep checked in - needed by flatcc to generate binary schema.
+
+NOTE TO CONTRIBUTORS: DO NOT EDIT THESE FILES BY HAND
+
+If you need to change anything here, it is done in the code generator,
+possibly followed by running `reflection/generate_code.sh` from the
+project root. But please only do this for testing do not include the
+generated files in a pull request unless agreed otherwise, and if so,
+do it in a separate commit.
+
+Normally new reflection code is generated during a release which also
+updates the version number in comments and there is no reason to update
+reflection on every commit unless it breaks something fundamentally.
+
+There is a build option `FLATCC_REFLECTION` to disable reflection which
+is helpful while making changes that affect the content of these files
+in a way that would prevent the flatcc compiler from building.
+
