Add a new PULL_REQUEST_TEMPLATE to the repo

[ci skip]

.github/PULL_REQUEST_TEMPLATE.md

@@ -0,0 +1,15 @@
+Before submitting your pull request be sure that:
+
+1. You haven't touched any file in the `spyderlib/defaults` directory.
+  - There is *absolutely* no need to touch those files.
+  - If you want to add new configuration options, please go to
+    `spyderlib/config/main.py`.
+2. You haven't eliminated unnecessary blank lines or spaces during your work.
+   That makes our reviewing work harder and it could introduce unnecessary
+   conflicts with other pull requests.
+3. You haven't added new icons to Spyder. Please leave decisions about what
+   icons to use to us :)
+
+----
+
+*Note*: You can safely remove this text before submitting your work.