3rd try at getting pt-PT PR #690 to merge

[n2ygk]

Fixes #690, #919

Description of the Change

Due to some confusion around ability to merge #690 this PR is my 2nd attempt. I've taken #690 and made some changes:

• Added @srtab's name to AUTHORS
• Updated the CHANGELOG
• Removed unnecessary header comments from LC_MESSAGES/django.po
• Updated docs/contributing.rst to describe how to add additional translations.

I don't know if there's a need for unit tests to confirm the translations are working as expected.

Checklist

• PR only contains one change (considered splitting up PR)
• unit-test added
• documentation updated
• CHANGELOG.md updated (only for user relevant changes)
• author name in AUTHORS

[codecov]

Codecov Report

Merging #920 (f21c0f4) into master (b9083eb) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #920   +/-   ##
=======================================
  Coverage   95.04%   95.04%           
=======================================
  Files          25       25           
  Lines        1070     1070           
=======================================
  Hits         1017     1017           
  Misses         53       53           

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