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`