Update CONTRIBUTING.md

.github/CONTRIBUTING.md

@@ -22,6 +22,7 @@ address things. Many issues have been around for a long time.
 * Make sure you're using a special branch just for this pull request. (Sometimes people unknowingly use a default branch, then later update that branch, which updates the pull request with the other changes if it hasn't been merged yet.)
 * Do NOT update the version number in the file. (This just causes conflicts.)
 * Do add your name to the list of contributors. (Don't worry about the formatting.) I'll try to remember to add it if you don't, but I sometimes forget as it's an extra step.
+* Your change needs to compile as both C and C++. Pre-C99 compilers should be supported (e.g. declare at start of block)
 
 # Specific libraries