Add pull request template.

.github/pull_request_template.md

+**DELETE ALL OF THIS TEXT BEFORE SUBMITTING**
+
+Thanks for your contribution! Before submitting this pull request, make sure you're okay with your code
+being put into the public domain. By submitting this pull request we will assume you're agreeing this.
+
+In addition, please ensure you're respecting the goals of the project. Your pull request will be rejected
+if you do any of the following:
+
+  * Split your code into multiple files
+  * Include a dependency on an external library
+  * Include copyrighted code
+  
+Your pull request may still be rejected for other reasons to those listed above, but we will inform you of
+the reasons.
\ No newline at end of file