Add require_changelog workflow action (#1051)

This action checks that an update was made to any file in docs/change_log/
but only if changes were made to files in markdown/. Presumably,
any changes outside of markdown/ do not affect the behavior and do not
require a notation in the change_log.

If the proper permissions are available, a comment is added to the PR informing
the PR author that an update to the changelog is missing. However, any PR's from
forks do not have permission on the pull_request event. Unfortunately, the
pull_request_target event doesn't seem to work at all. However, as the action
only attempts to add a comment on failure, we only get a failure in the correct
conditions, even if the error message is related to auth.

Author: waylan

.github/workflows/process.yml

@@ -0,0 +1,24 @@
+name: bot
+
+on:
+  pull_request:
+    branches:
+    - '**'
+
+jobs:
+  require_changelog:
+
+    runs-on: ubuntu-latest
+    steps:
+    - uses: actions/checkout@v2
+    - uses: mskelton/changelog-reminder-action@v1
+      with:
+        # Match any file in the docs/change_log/ dir.
+        changelogRegex: "docs/change_log/.*"
+        # Only require changelog update if changes were made in markdown/
+        include: "markdown/.*"
+        message: |
+          @${{ github.actor }}, thank you for your contribution. It appears that you have not added a comment to the
+          change log describing the changes you have made. Doing so will help to ensure your contribution is accepted.
+
+          Please see the [Contributing Guide](https://python-markdown.github.io/contributing/#pull-requests) for details.