docs: upate contributing guide to be more accurate

Spotted two small "imperfections" in CONTRIBUTING.md guide while reading them.

- `git remote add` raises and error: remote upstream already exists. That's because GitHub adds them for forked repositories.

- `npm install` had an explanation, but `npm run build` did not.

CONTRIBUTING.md

@@ -12,11 +12,11 @@ Next, clone your fork onto your computer (replacing YOUR_USERNAME with your actu
 git clone [email protected]:YOUR_USERNAME/web.git
 ```
 
-Once cloning is complete, change directory to the repository and add the Modern Web project as a remote.
+Once cloning is complete, change directory to the repository.
 
 ```sh
 cd web
-git remote add upstream [email protected]:modernweb-dev/web.git
+git remote -v # GitHub should add upstream remote for forked repos
 ```
 
 ## Preparing Your Local Environment for Development
@@ -28,7 +28,7 @@ npm install
 npm run build
 ```
 
-This will download and install all packages needed.
+This will download and install all packages needed, and prepares for local development.
 
 ## Making Your Changes