add a request for PR-squashing in contributing.md

CONTRIBUTING.md

@@ -1,13 +1,25 @@
-# How to Contribute
+# How to contribute
 
-## Contributor License Agreement
+## Single-change commits and pull requests
+
+A git commit ought to be a self-contained, single change with a descriptive
+message. This helps with review and with identifying or reverting changes if
+issues are uncovered later on.
+
+**Pull requests typically comprise a single git commit.** (In some cases, for
+instance for large refactors or internal rewrites, they may contain several.)
+In preparing a pull request for review, you may need to squash together
+multiple commits. We ask that you do this prior to sending the PR for review if
+possible. The `git rebase -i` command might be useful to this end.
+
+## Contributor license agreement (CLA)
 
 Contributions to this project must be accompanied by a Contributor License
 Agreement. You (or your employer) retain the copyright to your contribution;
 this simply gives us permission to use and redistribute your contributions as
 part of the project. Head over to <https://cla.developers.google.com/> to see
 your current agreements on file or to sign a new one.
 
-You generally only need to submit a CLA once, so if you've already submitted one
-(even if it was for a different project), you probably don't need to do it
+You generally only need to submit a CLA once, so if you've already submitted
+one (even if it was for a different project), you probably don't need to do it
 again.