CI: Improve pull request template

Trying to make it clearer that
* testing locally is an option
* DCO is required

Signed-off-by: Jussi Kukkonen <jkukkonen@google.com>

.github/PULL_REQUEST_TEMPLATE.md

@@ -1,13 +1,22 @@
-Please fill in the fields below to submit a pull request.  The more information
-that is provided, the better.
+<!--
+Before submitting a pull request:
+  * Please run linter and tests locally: "tox"
+  * Ensure your commits are signed-off-by: Use "commit --signoff"
+  * For more info, see docs/CONTRIBUTING.rst
 
-Fixes #<ISSUE NUMBER>
+Once commits are signed off and tested, describe the purpose and contents
+of the pull request below.
+-->
 
 **Description of the changes being introduced by the pull request**:
 
-**Please verify and check that the pull request fulfills the following
-requirements**:
 
+
+
+Fixes #<ISSUE NUMBER>
+
+
+<!-- Please check relevant options (you can remove the ones that do not apply) -->
 - [ ] The code follows the [Code Style Guidelines](https://github.com/secure-systems-lab/code-style-guidelines#code-style-guidelines)
 - [ ] Tests have been added for the bug fix or new feature
 - [ ] Docs have been added for the bug fix or new feature