Add GitHub build status badge to README.rst

[jdufresne]

Changelog-friendly one-liner:

Add GitHub build status badge to README.rst

Contributor checklist

• Provided the tests for the changes. (n/a)
• Gave a clear one-line description in the PR (that the maintainers can add to CHANGELOG.md on release).
• Assign the PR to an existing or new milestone for the target version (following Semantic Versioning).

[jdufresne]

Follow the link to preview the badge:

https://github.com/jazzband/pip-tools/workflows/CI/badge.svg

[codecov]

Codecov Report

Merging #1224 (5c06d11) into master (61eb515) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master    #1224   +/-   ##
=======================================
  Coverage   99.50%   99.50%           
=======================================
  Files          36       36           
  Lines        2855     2855           
  Branches      332      332           
=======================================
  Hits         2841     2841           
  Misses          8        8           
  Partials        6        6           

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[atugushev]

Thanks @jdufresne! Superseded by #1229.