Remind contributors not to force-push

Signed-off-by: Dave Rodgman <dave.rodgman@arm.com>

.github/pull_request_template.md

@@ -18,3 +18,10 @@ Please tick as appropriate and edit the reasons (e.g.: "backport: not needed bec
 
 Please refer to the [contributing guidelines](https://github.com/Mbed-TLS/mbedtls/blob/development/CONTRIBUTING.md), especially the
 checklist for PR contributors.
+
+Help make review efficient:
+* Multiple simple commits
+  - please structure your PR into a series of small commits, each of which does one thing
+* No force-push
+  - please do not force-push to update your PR - just add new commit(s)
+  - use `git merge` (or the GitHub web interface) to resolve conflicts - not `git rebase`