docs(contributing): add yarn before running jest

Add yarn before jest to grab the local binary of jest from node_modules to run individual tests
ISSUES CLOSED: #775

Author: anshumanv

.github/CONTRIBUTING.md

@@ -49,9 +49,7 @@ npm link webpack-cli
 -`BIN_TEST_CASES_GREP=/myCase jest test/BinTestCases.test.js`
 
 * To test a single CLI (other type of) test case:
-- If jest is not installed globally
--`npm i -g jest`
--`jest path/to/my-test.js`
+-`yarn jest path/to/my-test.js`
 
 * To test linting:
 -`npm run lint && npm run tslint`
@@ -74,9 +72,7 @@ yarn link webpack-cli
 -`BIN_TEST_CASES_GREP=/myCase jest test/BinTestCases.test.js`
 
 * To test a single CLI (other type of) test case:
-- If jest is not installed globally
- - `yarn global add jest`
--`jest path/to/my-test.js`
+-`yarn jest path/to/my-test.js`
 
 * To test linting:
 -`yarn lint && yarn tslint`