Added a pull request template. (#196)

This will shift the burden of keeping the changelog up-to-date from the maintainers to PR submitters, which will help good PR's to get released more quickly.

Author: nedtwigg

PULL_REQUEST_TEMPLATE.md

@@ -0,0 +1,10 @@
+After creating the PR, please add a commit that adds a bullet-point under the `-SNAPSHOT` section of [CHANGES.md](https://github.com/diffplug/spotless/blob/master/CHANGES.md) and [plugin-gradle/CHANGES.md](https://github.com/diffplug/spotless/blob/master/plugin-gradle/CHANGES.md) which includes:
+
+- [ ] a summary of the change
+- [ ] a link to the newly created PR
+
+If your change only affects a build plugin, and not the lib, then you only need to update the `CHANGES.md` for that plugin.
+
+If your change affects lib in an end-user-visible way (fixing a bug, updating a version) then you need to update `CHANGES.md` for both the lib and the build plugins.  Users of a build plugin shouldn't have to refer to lib to see changes that affect them.
+
+This makes it easier for the maintainers to quickly release your changes :)