Add documentation, see phetsims/chipper#1369

doc/how-to-change-a-string-key.md

@@ -67,6 +67,7 @@ keys, and several can be changed at once if necessary.
   [Admin Guide](https://github.com/phetsims/rosetta/blob/master/doc/admin-guide.md) for information on how to do this).
 - [ ] Change the value of the string key on master. This will need to be done in the string file and at all usage
   sites.
+- [ ] Run `grunt update` in the repo with the changed string to update the conglomerate string files.
 - [ ] Build the sim and verify that there are no string errors, then test the built sim and verify the string looks
   correct.
 - [ ] Change the value of the string key on the release branch. Again this will need to be done in the string file and