Add CLA checkbox to PR template

.github/pull_request_template.md

@@ -1,13 +1,13 @@
-<!--
-Thank you for pull request.
-Below are a few things we ask you kindly to self-check before getting a review. Remove checks that are not relevant.
--->
+## Related Issues
 
-Checklist
+<!-- Link any related issues, e.g. "Closes #123" or "See #456". -->
 
-* [ ] Used a [personal fork](https://docs.github.com/en/free-pro-team@latest/github/getting-started-with-github/fork-a-repo) of the repository to propose changes.
-* [ ] [Built the specification](https://github.com/modelica/fmi-standard/blob/master/CONTRIBUTING.adoc#building-the-specification-document).
-* [ ] [Re-generated schema figures](https://github.com/modelica/fmi-standard/blob/master/CONTRIBUTING.adoc#changing-the-xsd-schemas).
-* [ ] [Linted the documents](https://github.com/modelica/fmi-standard/blob/master/CONTRIBUTING.adoc#building-the-specification-document).
+## Changes
+
+<!-- A clear and concise description of what this PR changes and why. -->
 
-Add your [meaningful description](https://github.com/modelica/fmi-standard/blob/master/CONTRIBUTING.adoc#bug-reports) here and reference any related issues with `#<issue>`.
+## Checklist
+
+* [ ] Used a [personal fork](https://docs.github.com/en/free-pro-team@latest/github/getting-started-with-github/fork-a-repo) of the repository to propose changes.
+* [ ] [Built the specification](https://github.com/modelica/fmi-standard/blob/main/CONTRIBUTING.md#building-the-specification-document).
+* [ ] [Linted the documents](https://github.com/modelica/fmi-standard/blob/main/CONTRIBUTING.md#validating-the-files).