"yarn" command to install modules is not supported when building vs code [Updation of How To Contribute.md]

[vp007-dev]

Error when running "yarn":
*** Seems like you are using "yarn" which is not supported in this repo any more, please use "npm i" instead. ***

But:
when running npm i it succeeds without any problem.

Steps to Reproduce:

1. git clone https://github.com/microsoft/vscode.git
2. run yarn (according to contribution guidelines)

[deepak1556]

The docs have been updated, this is part of #196795