Add PR templates

First tell users to sign the CLA and then hint the correct description structure

Author: nicolasstucki

.github/PULL_REQUEST_TEMPLATE/fix-issue.md

@@ -0,0 +1,19 @@
+---
+name: "\U0001F527 Fix issue"
+about: Open a new pull request for the Dotty Compiler that fixes an issue from the issue tracker
+title: 'Fix #XYZ: A SHORT FIX DESCRIPTION'
+assignees: ''
+
+---
+
+<!-- 
+  TODO first sign the CLA 
+  https://www.lightbend.com/contribute/cla/scala
+-->
+
+## Fix #XYZ
+
+<!-- TODO description of the change --> 
+
+
+<!-- Ideally should have a called "Fix #XYZ: A SHORT FIX DESCRIPTION" -->

.github/PULL_REQUEST_TEMPLATE/other-pr.md

@@ -0,0 +1,19 @@
+---
+name: "Open a PR"
+about: Open a new pull request for the Dotty Compiler
+title: 'A SHORT DESCRIPTION'
+assignees: ''
+
+---
+
+<!-- 
+  TODO first sign the CLA 
+  https://www.lightbend.com/contribute/cla/scala
+-->
+
+## Description
+
+<!-- TODO description of the change --> 
+
+
+<!-- Ideally should have a single commit -->