Add a PR template. (#2344)

In order to remind to attach tests and to not commit any of the distribution file.

.github/PULL_REQUEST_TEMPLATE.md

@@ -0,0 +1,10 @@
+<!--
+
+Thank you for contributing!
+
+Please make sure to:
+ 
+- provide tests with your code changes to ensure they are working as expected.
+- not commit any of the distribution file (`c3.js`, `c3.css`, `c3.min.js`, `c3.min.css`).
+
+-->