Add pull request template

Author: garydgregory

.github/pull_request_template.md

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+
+Thanks for your contribution to [Apache Commons](https://commons.apache.org/)! Your help is appreciated!
+
+Before you push a pull request, review this list:
+
+- [ ] Read the [contribution guidelines](CONTRIBUTING.md) for this project.
+- [ ] Run a successful build using the default [Maven](https://maven.apache.org/) goal with `mvn`; that's `mvn` on the command line by itself.
+- [ ] Write unit tests that match behavioral changes, where the tests fail if the changes to the runtime are not applied. This may not always be possible but is a best-practice.
+- [ ] Write a pull request description that is detailed enough to understand what the pull request does, how, and why.
+- [ ] Each commit in the pull request should have a meaningful subject line and body. Note that commits might be squashed by a maintainer on merge.