[docs] Clarify on how to use the local distribution in the CONTRIBUTI…

…NG file (#13312)

In this commit, we are clarifying how to install the local distribution 
of Material-UI to test changes on a real case scenario.

It turns out that, if you don't point the instalation to the /build 
folder, when importing a Material-UI component, the path 
"@material-ui/core/<ComponentName>" does not work.

It took me a while to realize how to install the local distribution, 
that's why I decided to update the documentation.

CONTRIBUTING.md

@@ -161,7 +161,7 @@ Then, you can add Material-UI to any project you want to try your changes:
 
 ```shell
 # From the root folder of any project
-yarn add file:/path/to/material-ui/folder
+yarn add file:<path-to-material-ui-project>/packages/material-ui/build
 ```
 
 Now, every time you import `material-ui` in your project, it is going to use your local distribution.