docs(contributing): update instructions to run individual tests

Updated instructions to run individual tests in contributing.md
ISSUES CLOSED: #775

Author: anshumanv

.github/CONTRIBUTING.md

@@ -52,6 +52,8 @@ npm link webpack-cli
 -`npx jest path/to/my-test.js`
 - You can also install jest globally and run tests without npx.
 -`npm i -g jest`
+- If you have jest installed globally.
+-`jest path/to/my-test.js`
 
 * To test linting:
 -`npm run lint && npm run tslint`