Prepare release

Author: rlaboiss

RELEASE.md

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+Release type: major
+
+This version introduces support for the reStructuredText markup format. Thanks to Mark Shroyer for implementing it. The plugin documentation in the `README.md` file has been updated accordingly. Additionally, the documentation's wording and grammar have been improved.
+
+Non-user-visible improvements:
+- Compliance with Python linters' recommendations
+- Improved argument processing of the setUp() method of the unit tests
+- Using BeautifulSoup instead of regular expressions when asserting the results of the unit tests