Add git tags for releases of the plugin on wp org

[versusbassz]

When I look at git tree of this repo it often isn't obvious when previous releases were done.

Using of git tags would be very convenient.

Example of tagging for 3.31.8 realse:

git tag -a -m "3.31.8" 3.31.8 <commit_hash>
git push origin --tags

[versusbassz]

Version 3.31.9 has been released without a git tag also.
The "changelog" section in readme.txt hasn't been updated for versions: 3.31.8, 3.31.9.