readme.md: reviewer's guide: check for PR completeness (i.e. nothing is missing)

Readme.md

@@ -177,6 +177,7 @@ Reviewing changes to `src/*` and `nuklear.h`:
 
 * Ensure C89 compatibility.
 * The code should work for several backends to an acceptable degree.
+* Check no other parts of `nuklear.h` are related to the PR and thus nothing is missing.
 * Recommend simple optimizations.
   * Pass small structs by value instead of by pointer.
   * Use local buffers over heap allocation when possible.