docs: add .env.local instructions to readme

The dapp won't boot up without setting
`process.env.REACT_APP_PROJECT_ID`

dapps/react/react-dapp-v2/README.md

@@ -1,5 +1,19 @@
 # WalletConnect React App
 
+## Getting Started
+
+Install the app's dependencies:
+
+```bash
+yarn
+```
+
+Set up your local environment variables by copying the example into your own `.env.local` file:
+
+```bash
+cp .env.local.example .env.local
+```
+
 ## Develop
 
 ```bash