Add PR template

Author: lord

CONTRIBUTING.md renamed to .github/CONTRIBUTING.md

@@ -0,0 +1,5 @@
+Make sure you've checked off all these things before submitting:
+
+- [ ] This pull request isn't for a company's fork, it's intended for the upstream Slate shared by everybody.
+- [ ] This pull request is submitted to the `dev` branch.
+- [ ] If it makes frontend changes, this pull request has been tested in the latest version of Firefox, Chrome, IE, and Safari.