add pull request template

Author: edwardchalstrey1

.github/pull_request_template.md

@@ -0,0 +1,24 @@
+_Thanks for contributing to Gambit! Before you submitting or reviewing a pull request, check out our [guidelines for contributing](https://gambitproject.readthedocs.io/en/latest/developer.contributing.html)._
+
+_The person submitting the PR should update the headers below, before marking the PR as ready for review and assigning reviewers._
+
+### Issues closed by this PR
+
+_Add any issues that are being closed by this PR to the list. Use "Closes" or [another keyword](https://docs.github.com/en/issues/tracking-your-work-with-issues/using-issues/linking-a-pull-request-to-an-issue#linking-a-pull-request-to-an-issue-using-a-keyword) followed by the issue number._
+
+- Closes #ISSUE_NUMBER
+
+### Description of the changes in this PR
+
+_Update the description of the changes made in this PR. You can delete this section if the changes are already fully described in the linked issues._
+
+This PR ...
+
+### How to review this PR
+
+_The gambit repository contains source code and documentation for several different components. Explain how a reviewer should approach reviewing the specific changes made in this PR._
+
+_For example, you might include instructions like:_
+- _"Review code changes and ensure tests cover edge cases"_
+- _"Rebuild the GUI from this branch, then test features X, Y, Z work as expected"_
+- _"Click the link to the documentation page and sense check by reading"_