Add GitHub page link to readme

[Lee-W]

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Types of changes

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• Documentation Update

Description

Add GitHub page link to README

Checklist:

• Add test cases to all the changes you introduce
• Run ./script/lint 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

Steps to Test This Pull Request

1. poetry run mkdoc serve
2. Open http://127.0.0.1:8000

Expected behavior

See a link to github page (View full documentation)

Related Issue

#196

Additional context

Add any other context or screenshots about the pull request here.

[codecov]

Codecov Report

Merging #198 into master will not change coverage.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master     #198   +/-   ##
=======================================
  Coverage   92.14%   92.14%           
=======================================
  Files          35       35           
  Lines         968      968           
=======================================
  Hits          892      892           
  Misses         76       76           

Flag	Coverage Δ
#unittests	92.14% <100.00%> (ø)

Impacted Files	Coverage Δ
commitizen/commands/init.py	74.24% <100.00%> (ø)

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