Improve PULL_REQUEST_TEMPLATE

Make it easier for contributors to mix the text provided by GitHub
from the commit messages with the template content.

Rework the checklist to follow the style we use in loopback-next.

Signed-off-by: Miroslav Bajtoš <mbajtoss@gmail.com>

.github/PULL_REQUEST_TEMPLATE.md

@@ -1,25 +1,18 @@
-### Description
-
-
-#### Related issues
-
 <!--
-Please use the following link syntaxes:
+Please provide a high-level description of the changes made by your pull request.
 
-- connect to #49 (to reference issues in the current repository)
-- connect to strongloop/loopback#49 (to reference issues in another repository)
+Include references to all related GitHub issues and other pull requests, for example:
+
+Fixes #123
+Implements #254
+See also #23
 -->
 
-- connect to <link_to_referenced_issue>
+## Checklist
 
-### Checklist
-
-<!--
-- Please mark your choice with an "x" (i.e. [x], see
-https://github.com/blog/1375-task-lists-in-gfm-issues-pulls-comments)
-- PR's without test coverage will be closed.
--->
+👉 [Read and sign the CLA (Contributor License Agreement)](https://cla.strongloop.com/agreements/strongloop/loopback-connector) 👈
 
+- [ ] `npm test` passes on your machine
 - [ ] New tests added or existing tests modified to cover all changes
-- [ ] Code conforms with the [style
-  guide](http://loopback.io/doc/en/contrib/style-guide.html)
+- [ ] Code conforms with the [style guide](https://loopback.io/doc/en/contrib/style-guide-es6.html)
+- [ ] Commit messages are following our [guidelines](https://loopback.io/doc/en/contrib/git-commit-messages.html)