QA-365: Update the PR template to go with conventional commits

Changelog: None
Signed-off-by: Ole Petter <ole.orhagen@northern.tech>

.github/PULL_REQUEST_TEMPLATE.md

@@ -3,7 +3,19 @@
 
 🚨 Please review the [guidelines for contributing](https://github.com/mendersoftware/mender/blob/master/CONTRIBUTING.md) to this repository.
 
-- [ ] Make sure that all commits have a [`Changelog`](https://github.com/mendersoftware/mender/blob/master/CONTRIBUTING.md#changelog-tags) tag. If nothing should be added to the Changelog, add a `Changelog: None` tag. If there is a change, add `Changelog: Commit`, or `Changelog: Title`, depending on what should be included in the changelog.
+- [ ] Make sure that all commits follow the conventional commit [specification](https://www.github.com/mendersoftware/mendertesting/commitlint/grammar.md) for the Mender project.
+
+The majority of our contributions are fixes, which means your commit should have
+the form below:
+
+```
+fix: <SHORT DESCRIPTION OF FIX>
+
+<OPTIONAL LONGER DESCRIPTION>
+
+Changelog: <USER-FRIENDLY-CHANGE-DESCRIPTION> or <None>
+Ticket: <TICKET NUMBER> or <None>
+```
 
 - [ ] Make sure that all commits are signed with [`git --signoff`](https://git-scm.com/book/en/v2/Git-Tools-Signing-Your-Work). Also note that the signoff author must match the author of the commit.