message: update CONTRIBUTE guide: verify local build
  included new step in CONTRIBUTE with instructions for verifying yarn local build
diff: @@ -35,7 +35,14 @@ git push origin fix/some-bug-#123
  4) Go to [github.com/beefyfinance/beefy-api](https://github.com/beefyfinance/beefy-api) in your web browser and issue a new pull request.
  -5) Maintainers will review your code and possibly ask for changes before your code is pulled in to the main repository. We'll check that all tests pass, review the coding style, and check for general code correctness. If everything is OK, we'll merge your pull request and your code will be part of Beefy's API.
  +5) Verify your local build succeeds.
  +
  +```
  +yarn
  +yarn start
  +```
  +
  +6) Maintainers will review your code and possibly ask for changes before your code is pulled in to the main repository. We'll check that all tests pass, review the coding style, and check for general code correctness. If everything is OK, we'll merge your pull request and your code will be part of Beefy's API.
  *IMPORTANT* Please pay attention to the maintainer's feedback, since its a necessary step to keep up with the standards Beefy.Finance attains to.
  