doc: add some use info to CONTRIBUTING.md

[mhdawson]

Adding some information that would have been useful
to me when I was working on my first commit.

[mhdawson]

Note that I suggested npm run check as opposed to npm run test at the top level since the tests don't seem to all pass.

[codecov-io]

Codecov Report

Merging #531 into master will decrease coverage by 0.13%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master     #531      +/-   ##
==========================================
- Coverage   95.15%   95.01%   -0.14%     
==========================================
  Files         147      147              
  Lines        9861     9505     -356     
  Branches      541      532       -9     
==========================================
- Hits         9383     9031     -352     
+ Misses        478      474       -4

Impacted Files	Coverage Δ
src/common-utils.ts	75% <0%> (-20.24%)	⬇️
test/test-stackdriver-cloudtrace.ts	92% <0%> (-6.96%)	⬇️
test/test-stackdriver-monitoring-utils.ts	93.33% <0%> (-6.67%)	⬇️
test/test-zipkin.ts	91.66% <0%> (-6.07%)	⬇️
test/test-tracecontext-format.ts	98.76% <0%> (-0.15%)	⬇️
...c/zpages-frontend/controllers/tracez.controller.ts	100% <0%> (ø)	⬆️
...es-frontend/controllers/traceconfigz.controller.ts	100% <0%> (ø)	⬆️
src/zpages-frontend/controllers/rpcz.controller.ts	100% <0%> (ø)	⬆️
src/zpages-frontend/routes.ts	100% <0%> (ø)	⬆️
src/validators.ts	100% <0%> (ø)	⬆️
... and 4 more

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[mayurkale22]

Thanks for the PR, LGTM!