Add a pull request template

Again, not entirely a fan of these, but there's been several PRs
which would have been better off targeting the original repo.

Author: SquidDev

.github/pull_request_template.md

@@ -0,0 +1,9 @@
+<!--
+Unless this feature is specific to CC:Tweaked, try to [target the original ComputerCraft repo](https://github.com/dan200/ComputerCraft/) instead. There's a lot more people watching it, so it allows the wider community to contribute.
+-->
+
+## Useful information to include:
+ - Brief explanation of the changes you've made.
+ - Rationale of why this change has been made/reasoning behind it.
+
+The more information you can provide, the easier it is to review something now _and_ to see why a change was made, when the code needs updating in the future.