Update PR Template to include squashing commits

.github/PULL_REQUEST_TEMPLATE.md

@@ -9,3 +9,6 @@ To ensure your PR is dealt with swiftly please check the following:
 - [ ] Each table column should be padded with one space on either side
 - [ ] You have searched the repository for any relevant issues or pull requests
 - [ ] Any category you are creating has the minimum requirement of 3 items
+- [ ] All changes have been [squashed][squash-link] into a single commit
+
+[squash-link]: <https://github.com/todotxt/todo.txt-android/wiki/Squash-All-Commits-Related-to-a-Single-Issue-into-a-Single-Commit>