Introduce the pull-request template. (#2784)

* Introduce the pull-request template.

This  change xplicitly sets expectations from pull requests.

Reason:
* Highlight the need in autotests and JIRA issues for bugfixes
* ask PR creators to propose the changelog entries for their changes.
* Provide hints about referencing people

* Move the pull request template to .github

.github/PULL_REQUEST_TEMPLATE.md

+# Description
+
+See [JENKINS-XXXXX](https://issues.jenkins-ci.org/browse/JENKINS-XXXXX).
+
+Details: TODO
+
+<!-- Comment: 
+If the issue is not fully described in the ticket, please put additional comments (justification, pull request links, etc.).
+-->
+
+### Changelog entries
+
+Proposed changelog entries:
+
+* Entry 1: Issue, Human-readable Text
+* ...
+
+<!-- Comment: 
+The changelogs will be integrated by the core maintainers after the merge.  See the changelog examples here: https://jenkins.io/changelog/ -->
+
+### Submitter checklist
+
+- [ ] JIRA issue is well described
+- [ ] Link to JIRA ticket in description, if appropriate
+- [ ] Appropriate autotests or explanation to why this change has no tests
+- [ ] For new API and extension points: Link to the reference implementation in open-source (or example in Javadoc)
+
+<!-- Comment: 
+ * We do not require JIRA issues for minor improvements.
+ * Bugfixes should have a JIRA issue (backporting process).
+ * Major new features should have a JIRA issue reference.
+-->
+
+### Desired reviewers
+
+@mention
+
+<!-- Comment:
+If you want to get reviews from particular people, please CC them.
+If you need an accelerated review process by the community (e.g., for critical bugs), mention @jenkinsci/code-reviewers
+-->