Replace NPM with Yarn in quick start guide

[cravend]

Description

The latest version of create-react-app uses Yarn as the package manager, not NPM (even though it's invoked with npx). Because of this, specifying npm install as step 2 of the quick start will likely cause confusion for new learners.

Fix

This changes npm install to yarn add in the quick start guide. I don't know if there are any other installation guides in the docs; I took a look and didn't see any but it's possible I missed something.

[pshrmn]

From what I've generally seen in documentation, yarn add is included as a supplementary way to install, not exclusive.

# NPM
npm install react-router-dom

# Yarn
yarn add react-router-dom

[cravend]

Yeah, that's an option too! The reason I didn't include both is because the documentation seems to be designed to used in one fluid motion (run create react app, move into the folder, then install react-router), so I figured anyone who deviated from that path (i.e. using NPM) will know enough to change the commands they run.

[timdorr]

@pshrmn That makes sense for a plain project, but this is coming after CRA installs, which will give you a yarn setup. So, running this command would give you two lockfiles (yarn.lock and package-lock.json), which might be confusing to some folks.

Thanks for the update!