update npm -> yarn, fix chokidar watch script to run tests, update re…

…adme contributing docs

README.md

@@ -175,6 +175,18 @@ DEBUG=cypress:webpack:stats
 
 Use the [version of Node that matches Cypress](https://github.com/cypress-io/cypress/blob/develop/.node-version).
 
+Build the typescript files:
+
+```shell
+yarn build
+```
+
+Watch the typescript files and rebuild on file change:
+
+```shell
+yarn build --watch
+```
+
 Run all tests once:
 
 ```shell

package.json

@@ -14,12 +14,12 @@
  "secure": "nsp check",
  "semantic-release": "semantic-release",
  "size": "npm pack --dry",
- "pretest": "npm run lint && npm run build",
- "test": "npm run test-unit && npm run test-e2e",
+ "pretest": "yarn lint && yarn build",
+ "test": "yarn test-unit && yarn test-e2e",
  "test-debug": "node --inspect --debug-brk ./node_modules/.bin/_mocha",
  "test-e2e": "mocha test/e2e/*.spec.js",
  "test-unit": "mocha test/unit/*.spec.js",
- "test-watch": "chokidar '*.js' 'test/unit/*.js' -c 'npm run test-unit'",
+ "test-watch": "yarn test-unit & chokidar '*.js' 'test/unit/*.js' -c 'yarn test-unit'",
  "types": "tsc --noEmit"
  },
  "husky": {