Improve PULL_REQUEST_TEMPLATE.md (#323)

- Make the sections a bit smaller.
- Add more explanation when linking issues.

<!-- Thank you for submitting a Pull Request. If you're new to
contributing to BCApps please read our pull request guideline below
* https://github.com/microsoft/BCApps/Contributing.md
-->
#### Summary <!-- Provide a general summary of your changes -->

Improve the PR template to pretty much look like this one.

.github/PULL_REQUEST_TEMPLATE.md

@@ -1,7 +1,7 @@
 <!-- Thank you for submitting a Pull Request. If you're new to contributing to BCApps please read our pull request guideline below
 * https://github.com/microsoft/BCApps/Contributing.md
 -->
-# Summary <!-- Provide a general summary of your changes -->
+#### Summary <!-- Provide a general summary of your changes -->
 
-# Work Item(s) <!-- Add the issue number here after the # -->
-Fixes #
+#### Work Item(s) <!-- Add the issue number here after the #. The issue needs to be open and approved. Submitting PRs with no linked issues or unapproved issues is highly discouraged. -->
+Fixes #