chore: update dev environment guide to use yarn (#13800)

Author: crisbeto

DEV_ENVIRONMENT.md

@@ -1,18 +1,18 @@
 # Developer guide: getting your environment set up
 
 1. Make sure you have `node` installed with a version at _least_ 5.5.0.
-2. Run `npm install -g gulp` to install gulp.
-3. Fork the `angular/material2` repo. 
-4. Clone your fork. 
+2. Run `yarn global add gulp` to install gulp.
+3. Fork the `angular/material2` repo.
+4. Clone your fork.
    Recommendation: name your git remotes `upstream` for `angular/material2`
    and `<your-username>` for your fork. Also see the [team git shortcuts](https://github.com/angular/material2/wiki/Team-git----bash-shortcuts).
-5. From the root of the project, run `npm install`.
+5. From the root of the project, run `yarn`.
 
 
 To build Material in dev mode, run `gulp material:build`.
 To build Material in release mode, run `gulp material:build-release`
- 
-To bring up a local server, run `gulp serve:devapp`. This will automatically watch for changes 
+
+To bring up a local server, run `gulp serve:devapp`. This will automatically watch for changes
 and rebuild. The browser should refresh automatically when changes are made.
 
 ### Running tests