Update pull request template (#2528)

Co-Authored-By: hanzei <16541325+hanzei@users.noreply.github.com>

  * Move into .github
  * Update PR template
  * Update .github/PULL_REQUEST_TEMPLATE.md

.github/PULL_REQUEST_TEMPLATE.md

@@ -0,0 +1,28 @@
+<!-- Thank you for contributing a pull request! Here are a few tips to help you:
+
+1. If this is your first contribution, make sure you've read the Contribution Checklist https://developers.mattermost.com/contribute/getting-started/contribution-checklist/
+2. Read our blog post about "Submitting Great PRs" https://developers.mattermost.com/blog/2019-01-24-submitting-great-prs
+3. Take a look at other repository specific documentation at https://developers.mattermost.com/contribute
+-->
+
+#### Summary
+<!--
+A description of what this pull request does.
+-->
+
+#### Ticket Link
+<!--
+If this pull request addresses a Help Wanted ticket, please link the relevant GitHub issue, e.g.
+
+  Fixes https://github.com/mattermost/mattermost-server/issues/XXXXX
+
+Otherwise, link the JIRA ticket.
+-->
+
+#### Related Pull Requests
+<!--
+List all PRs related to resolving a ticket. For instance, if you submitted a PR to `mattermost/mattermost-server`, please include it here.
+-->
+- Has server changes (please link here)
+- Has redux changes (please link here)
+- Has mobile changes (please link here)