[docs] Add yarn install instructions in CONTRIBUTING.md

CONTRIBUTING.md

@@ -23,6 +23,10 @@ If you decide to fix an issue, please be sure to check the comment thread in cas
 
 If somebody claims an issue but doesn’t follow up for more than a week, it’s fine to take it over but you should still leave a comment.
 
+## Package Manager
+
+Use [yarn](https://yarnpkg.com/en/docs) as a package manager to deal with project dependencies.
+
 ## Sending a Pull Request
 
 Material-UI is a community project, so Pull Requests are always welcome, but, before working on a large change, it is best to open an issue first to discuss it with the maintainers.
@@ -31,7 +35,7 @@ When in doubt, keep your Pull Requests small. To give a Pull Request the best ch
 
 1. Fork the repository.
 
-2. Clone the fork to your local machine and add upstream remote
+2. Clone the fork to your local machine and add upstream remote:
 
 ```sh
 git clone git@github.com:<yourname>/material-ui.git
@@ -46,19 +50,30 @@ git checkout master
 git pull upstream master
 ```
 
-4. Create a new topic branch:
+4. Install dependencies:
 
 ```sh
-git checkout -b my-topic-branch
+yarn install
 ```
 
-5. Make changes, commit and push to your fork:
+or
+
+````sh
+yarn
+
+5. Create a new topic branch:
+
+```sh
+git checkout -b my-topic-branch
+````
+
+6. Make changes, commit and push to your fork:
 
 ```sh
 git push -u
 ```
 
-6. Go to [the repository](https://github.com/mui-org/material-ui) and make a Pull Request.
+7. Go to [the repository](https://github.com/mui-org/material-ui) and make a Pull Request.
 
 The core team is monitoring for Pull Requests. We will review your Pull Request and either merge it, request changes to it, or close it with an explanation.