LUCENE-8842: add a pull request template (#705)

* LUCENE-8842: add a pull request template to a new .github/ root-level directory

.github/PULL_REQUEST_TEMPLATE.md

@@ -0,0 +1,41 @@
+<!--
+Before creating a pull request, please file an issue in the ASF Jira system for Lucene or Solr:
+
+* https://issues.apache.org/jira/projects/LUCENE
+* https://issues.apache.org/jira/projects/SOLR
+
+You will need to create an account in Jira in order to create an issue.
+
+The title of the PR should reference the Jira issue number in the form:
+
+* LUCENE-####: <short description of problem or changes>
+* SOLR-####: <short description of problem or changes>
+
+LUCENE and SOLR must be fully capitalized. A short description helps people scanning pull requests for items they can work on.
+
+Properly referencing the issue in the title ensures that Jira is correctly updated with code review comments and commits. -->
+
+
+# Description
+
+Please provide a short description of the changes you're making with this pull request.
+
+# Solution
+
+Please provide a short description of the approach taken to implement your solution.
+
+# Tests
+
+Please describe the tests you've developed or run to confirm this patch implements the feature or solves the problem.
+
+# Checklist
+
+Please review the following and check all that apply:
+
+- [] I have reviewed the guidelines for [How to Contribute](https://wiki.apache.org/solr/HowToContribute) and my code conforms to the standards described there to the best of my ability.
+- [] I have created a Jira issue and added the issue ID to my pull request title.
+- [] I am authorized to contribute this code to the ASF and have removed any code I do not have a license to distribute.
+- [] I have developed this patch against the `master` branch.
+- [] I have run `ant precommit` and the appropriate test suite.
+- [] I have added tests for my changes.
+- [] I have added documentation for the Ref Guide (for Solr changes only).