github: improve template CLA request info

Indicate that the CLA only has to be signed once, as we have had
contributors think that was required for each new PR.

.github/PULL_REQUEST_TEMPLATE.md

@@ -1,7 +1,9 @@
 Make sure these boxes are signed before submitting your Pull Request -- thank you.
 
-- [ ] I have read the contributing guide lines at https://docs.suricata.io/en/latest/devguide/codebase/contributing/contribution-process.html
-- [ ] I have signed the Open Information Security Foundation contribution agreement at https://suricata.io/about/contribution-agreement/
+- [ ] I have read the contributing guide lines at
+   https://docs.suricata.io/en/latest/devguide/codebase/contributing/contribution-process.html
+- [ ] I have signed the Open Information Security Foundation contribution agreement at
+   https://suricata.io/about/contribution-agreement/ (note: this is only required once)
 - [ ] 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: