fix #820

.github/CONTRIBUTING.md

@@ -17,6 +17,7 @@ address things. Many issues have been around for a long time.
 
 # Pull requests
 
+* 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.