Contributing in README: Use yarn instead of NPM (#2603)

* docs(README): use yarn instead of npm

* docs(README): fix yarn test-watch to use flag

uses flag --watchAll to watch all tests for changes

* docs(README): fix coverage command

* docs(README): use yarn cover for tests and coverage report

* docs(README): remove watch tests instructions

watch tests occurs when you run `yarn test` and press `a` when prompted

* docs(README): re-add Watch tests

* docs(README): fix auto formatting

README.md

@@ -125,25 +125,25 @@ https://github.com/reactstrap/code-sandbox-examples
 Install dependencies:
 
 ```sh
-npm install
+yarn install
 ```
 
 Run examples at [http://localhost:8080/](http://localhost:8080/) with webpack dev server:
 
 ```sh
-npm start
+yarn start
 ```
 
 Run tests & coverage report:
 
 ```sh
-npm test
+yarn cover
 ```
 
 Watch tests:
 
 ```sh
-npm run test-watch
+yarn test
 ```
 
 ## Releasing