Docs: Clarify the instructions on starting the local environment for the first time.

This adds a mention of cloning the repository before running the npm commands in the terminal. 

Props justinahinon.
Fixes #52278. See #51414.

git-svn-id: https://develop.svn.wordpress.org/trunk@49957 602fd350-edb4-49c9-b593-d223f7449a82

Author: SergeyBiryukov

README.md

@@ -29,6 +29,8 @@ Ensure [Docker](https://www.docker.com/products/docker-desktop) is running befor
 
 #### To start the development environment for the first time
 
+Clone the current repository using `git clone https://github.com/WordPress/wordpress-develop.git`. Then in your terminal move to the repository folder `cd wordpress-develop` and run the following commands:
+
 ```
 npm install
 npm run build:dev