chore: update CONTRIBUTING.md to include code formatting

[yoshi-automation]

This PR was generated using Autosynth. 🌈

Synth log will be available here:
https://source.cloud.google.com/results/invocations/0ba8d2f8-7f35-417b-8c9a-d829ca01eac0/targets

Source-Link: googleapis/synthtool@5b48b07

[codecov]

Codecov Report

Merging #217 into master will decrease coverage by 0.07%.
The diff coverage is n/a.

@@             Coverage Diff              @@
##             master     #217      +/-   ##
============================================
- Coverage     67.73%   67.66%   -0.08%     
  Complexity      379      379              
============================================
  Files            36       36              
  Lines          1968     1976       +8     
  Branches        260      262       +2     
============================================
+ Hits           1333     1337       +4     
- Misses          527      531       +4     
  Partials        108      108              

Impacted Files	Coverage Δ	Complexity Δ
...re/src/main/java/com/google/cloud/BaseService.java	55.55% <0.00%> (-6.95%)	2.00% <0.00%> (ø%)
...va/com/google/cloud/http/HttpTransportOptions.java	56.94% <0.00%> (-1.63%)	10.00% <0.00%> (ø%)
...a/com/google/cloud/testing/BaseEmulatorHelper.java	31.21% <0.00%> (-0.19%)	5.00% <0.00%> (ø%)
...src/main/java/com/google/cloud/MetadataConfig.java	0.00% <0.00%> (ø)	0.00% <0.00%> (ø%)
...ud-core/src/main/java/com/google/cloud/Policy.java	92.11% <0.00%> (+0.03%)	20.00% <0.00%> (ø%)
...src/main/java/com/google/cloud/ServiceOptions.java	44.22% <0.00%> (+0.79%)	32.00% <0.00%> (ø%)
.../src/main/java/com/google/cloud/AsyncPageImpl.java	66.66% <0.00%> (+2.38%)	2.00% <0.00%> (ø%)

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