Add template for pull request descriptions

Closes #5258
// FREEBIE

.github/PULL_REQUEST_TEMPLATE.md

@@ -0,0 +1,18 @@
+<!-- You can remove this section if you have contributed before -->
+### First time contributor checklist
+<!-- mark with x between the brackets -->
+- [ ] I have read [how to contribute](https://github.com/WhisperSystems/Signal-Android/blob/master/CONTRIBUTING.md) to this project
+- [ ] I have signed the [Contributor Licence Agreement](https://whispersystems.org/cla/)
+
+### Contributor checklist
+<!-- mark with x between the brackets -->
+- [ ] I am following the [Code Style Guidelines](https://github.com/WhisperSystems/Signal-Android/wiki/Code-Style-Guidelines)
+- [ ] I have tested my contribution on these devices:
+ * Device A, Android X.Y.Z
+ * Device B, Android Z.Y
+- [ ] My contribution is fully baked and ready to be merged as is
+- [ ] I have made the choice whether I want the Bithub reward or not by omitting or adding the word `FREEBIE` in my commit message
+
+----------
+
+### Description