chore: add pull request template

Tetragon currently does not have a pull request template to provide contributors with directions on how to contribute to the project on Github.

This patch fixes this by adding:

1. pull_request_template.md file covering the important information contributors need to open request that checks that required criteria.

Fixes: #1929

Signed-off-by: Paul Arah <[email protected]>

.github/pull_request_template.md

@@ -0,0 +1,19 @@
+Please ensure your pull request adheres to the following guidelines:
+
+- [ ] For first time contributors, read [Submitting a pull request](https://tetragon.io/docs/contribution-guide/submitting-a-pull-request/).
+- [ ] All code is covered by unit and/or end-to-end tests tests where feasible.
+- [ ] All commits contain a well written commit message including a title,
+      description and a `Fixes: #XXX` line if the commit addresses a particular
+      GitHub issue.
+- [ ] All commits are signed off. See the section [Developer’s Certificate of Origin](https://docs.cilium.io/en/stable/contributing/development/contributing_guide/#dev-coo)
+- [ ] Provide a title or release-note blurb suitable for the release notes.
+- [ ] Are you a user of Tetragon? Please add yourself to the [Users doc](https://github.com/cilium/cilium/blob/main/USERS.md) in the Cilium repository. 
+- [ ] Thanks for contributing!
+
+<!-- Description of change -->
+
+Fixes: #issue-number
+
+```release-note
+<!-- Enter the release note text here if needed or remove this section! -->
+```