Create proofreader_membership.md (#514)

Author: ezio-melotti

.github/ISSUE_TEMPLATE/proofreader_membership.md

@@ -0,0 +1,21 @@
+---
+name: Python Proofreaders Team Membership
+about: Request to join the Python Proofreaders team
+title: "Request for Python Proofreaders team membership: name"
+---
+
+<!--
+Anyone can create this issue to request membership to join the Python Proofreaders team.
+
+More details: https://devguide.python.org/documentation/help-documenting/#proofreading
+-->
+
+# Request for Python Proofreaders team membership
+@python/organization-owners, please add a new proofreader:
+
+<!-- replace with real info -->
+
+|     | Description |
+| --- | -------------------------- |
+| GitHub username   | @your-github-username |
+| Additional info   | e.g. _"I've made 1 million PRs"_, _"I found over 9000 typos in the docs"_ |