Document fix for unauthorized push

[zhouzi]

This PR adds the solution to unauthorized push to the README, as initially reported in #744.

[peaceiris]

Thanks! I will use the following URL instead of saving the image under Git's control.

https://user-images.githubusercontent.com/30958501/168609359-242f59a9-34c6-4272-9793-d6b2cc823837.png

[codecov]

Codecov Report

Merging #752 (48d3b49) into main (cf3948a) will not change coverage.
The diff coverage is n/a.

❗ Current head 48d3b49 differs from pull request most recent head 874da69. Consider uploading reports for the commit 874da69 to get more accurate results

@@           Coverage Diff           @@
##             main     #752   +/-   ##
=======================================
  Coverage   61.45%   61.45%           
=======================================
  Files           4        4           
  Lines         275      275           
  Branches       45       45           
=======================================
  Hits          169      169           
  Misses        105      105           
  Partials        1        1           

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update cf3948a...874da69. Read the comment docs.

[peaceiris]

LGTM!

[peaceiris]

#753 is another solution.