PR template (#231)

Inspired by: https://ucsc-cgl.atlassian.net/browse/SEAB-4603

Author: Charles Overbeck

.github/PULL_REQUEST_TEMPLATE.md

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+**Description**
+A description of the PR, should include a decent explanation as to why this change was needed and a decent explanation as to what this change does
+
+**Issue**
+A link to a github issue or SEAB- ticket (using that as a prefix)
+
+Please make sure that you've checked the following before submitting your pull request. Thanks!
+
+- [ ] Ensure that the PR targets the correct branch. Check the milestone or fix version of the ticket.