Clean README of the WebUI

glances/outputs/static/README.md

 # How to contribute?
 
-In order to build the assets of the Web UI, you'll need [npm](https://docs.npmjs.com/getting-started/what-is-npm) or [yarn](https://yarnpkg.com).
+In order to build the assets of the Web UI, you'll need [npm](https://docs.npmjs.com/getting-started/what-is-npm).
 
 You must run the following command from the `glances/outputs/static/` directory.
 
 ## Install dependencies
 
-You can use yarn or npm:
-
-```bash
-$ yarn
-```
-
-or:
-
 ```bash
 $ npm install
 ```