add PyPI download count badge

[Lee-W]

Description

add PyPI download count per month to readme

Checklist

• Add test cases to all the changes you introduce
• Run ./script/format and ./script/test locally to ensure this change passes linter check and test
• Test the changes on the local machine manually
• Update the documentation for the changes

Expected behavior

Steps to Test This Pull Request

1. poetry run mkdocs build
2. poetry run mkdocs serve
3. Go to http://127.0.0.1

Additional context

[codecov]

Codecov Report

Merging #342 (055fc17) into master (1a33099) will not change coverage.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master     #342   +/-   ##
=======================================
  Coverage   97.20%   97.20%           
=======================================
  Files          35       35           
  Lines        1000     1000           
=======================================
  Hits          972      972           
  Misses         28       28           

Flag	Coverage Δ
unittests	97.20% <100.00%> (ø)

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Impacted Files	Coverage Δ
commitizen/cli.py	97.50% <ø> (ø)
commitizen/__version__.py	100.00% <100.00%> (ø)
commitizen/commands/bump.py	94.44% <100.00%> (ø)
commitizen/config/yaml_config.py	100.00% <100.00%> (ø)
commitizen/git.py	95.55% <100.00%> (ø)

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