Adding PR template

Author: jberkel

.github/PULL_REQUEST_TEMPLATE/template.md

@@ -0,0 +1,9 @@
+Thanks for taking the time to submit a pull request.
+
+Before submitting, please do the following:
+
+- Run `make lint` to check if there are any format errors (install [swiftlint](https://github.com/realm/SwiftLint#installation) first)
+- Run `swift test` to see if the tests pass.
+- Write new tests for new functionality.
+- Update documentation comments where applicable.
+