docs: Added pull request template

Author: edcarroll

.github/PULL_REQUEST_TEMPLATE.md

@@ -0,0 +1,6 @@
+Before submitting a pull request, please make sure you have at least performed the following:
+
+ - [] read and followed the [CONTRIBUTING.md](https://github.com/edcarroll/ng2-semantic-ui/blob/master/CONTRIBUTING.md#) guide.
+ - [] linted, built and tested the changes locally.
+ - [] added/updated any applicable API documentation.
+ - [] added/updated any applicable demos.