github: add pull request template

.github/PULL_REQUEST_TEMPLATE.md

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+Make sure these boxes are signed before submitting your Pull Request -- thank you.
+
+- [ ] I have read the contributing guide lines at https://redmine.openinfosecfoundation.org/projects/suricata/wiki/Contributing
+- [ ] I have signed the Open Information Security Foundation contribution agreement at https://suricata-ids.org/about/contribution-agreement/
+- [ ] I have updated the user guide (in doc/userguide/) to reflect the changes made (if applicable)
+
+Link to [redmine](https://redmine.openinfosecfoundation.org/projects/suricata/issues) ticket:
+
+Describe changes:
+-
+-
+-
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+[PRScript](https://redmine.openinfosecfoundation.org/projects/suricata/wiki/PRscript) output (if applicable):
+