docs: update CONTRIBUTING.md to include code formatting (#534)

[yoshi-automation]

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[googlebot]

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[codecov]

Codecov Report

Merging #346 into master will decrease coverage by 1.98%.
The diff coverage is n/a.

@@             Coverage Diff              @@
##             master     #346      +/-   ##
============================================
- Coverage     79.74%   77.75%   -1.99%     
- Complexity     1184     1185       +1     
============================================
  Files            77       77              
  Lines          5089     6177    +1088     
  Branches        563      665     +102     
============================================
+ Hits           4058     4803     +745     
- Misses          748     1019     +271     
- Partials        283      355      +72     

Impacted Files	Coverage Δ	Complexity Δ
...com/google/cloud/bigquery/GoogleSheetsOptions.java	77.14% <0.00%> (-12.86%)	10.00% <0.00%> (ø%)
...main/java/com/google/cloud/bigquery/ModelInfo.java	77.21% <0.00%> (-8.92%)	24.00% <0.00%> (ø%)
...in/java/com/google/cloud/bigquery/RoutineInfo.java	67.47% <0.00%> (-7.77%)	21.00% <0.00%> (ø%)
...google/cloud/bigquery/HivePartitioningOptions.java	83.78% <0.00%> (-6.22%)	11.00% <0.00%> (ø%)
...main/java/com/google/cloud/bigquery/DatasetId.java	81.25% <0.00%> (-5.42%)	13.00% <0.00%> (ø%)
.../com/google/cloud/bigquery/ConnectionProperty.java	91.89% <0.00%> (-5.26%)	12.00% <0.00%> (ø%)
.../google/cloud/bigquery/DatastoreBackupOptions.java	88.23% <0.00%> (-5.10%)	8.00% <0.00%> (ø%)
...google/cloud/bigquery/ExternalTableDefinition.java	57.69% <0.00%> (-3.77%)	19.00% <0.00%> (ø%)
...va/com/google/cloud/bigquery/InsertAllRequest.java	86.25% <0.00%> (-3.75%)	23.00% <0.00%> (ø%)
...in/java/com/google/cloud/bigquery/TableResult.java	85.29% <0.00%> (-3.60%)	15.00% <0.00%> (ø%)
... and 52 more

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