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`