docs(contributing): commands to install jest globally

Add commands to install jest globally for running individual tests

ISSUES CLOSED: #775

Author: anshumanv

CONTRIBUTING.md renamed to .github/CONTRIBUTING.md

@@ -49,6 +49,8 @@ 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` 
 
 * To test linting:
@@ -72,6 +74,8 @@ 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`
 
 * To test linting: