chore: regenerate README

[yoshi-automation]

This PR was generated using Autosynth. 🌈

Log from Synthtool

2020-10-16 00:16:46,565 synthtool [DEBUG] > Executing /root/.cache/synthtool/java-iamcredentials/.github/readme/synth.py.
On branch autosynth-readme
nothing to commit, working directory clean
2020-10-16 00:16:47,490 synthtool [DEBUG] > Wrote metadata to .github/readme/synth.metadata/synth.metadata.

Full log will be available here:
https://source.cloud.google.com/results/invocations/5838df64-ef18-4437-927c-238da7e3ccc3/targets

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[codecov]

Codecov Report

Merging #253 into master will not change coverage.
The diff coverage is n/a.

@@            Coverage Diff            @@
##             master     #253   +/-   ##
=========================================
  Coverage     66.76%   66.76%           
  Complexity       48       48           
=========================================
  Files             6        6           
  Lines           349      349           
  Branches          5        5           
=========================================
  Hits            233      233           
  Misses          111      111           
  Partials          5        5           

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