[flags-sdk/openfeature] Fix dependencies and update README (#1115)

- Adds `pnpm` to the README
- Adds `--legacy-peer-deps` to the `npm install` command
`npm` fails to install due to the canary version being used. The
`@vercel/analytics` requires Next.js >= 13, which it is, but `npm` does
not recognise the canary tag.
- Adds a step to install the Vercel CLI to the README

Author: AndyBitz

flags-sdk/openfeature/README.md

@@ -25,7 +25,19 @@ If you deployed your own instance of this example you can also use the [Flags Ex
 
 ### Step 1: Link the project
 
-In order to use the Flags Explorer, you need to link the project on your local machine.
+First, install the latest version of the Vercel CLI:
+
+```bash
+pnpm i -g vercel
+```
+
+or
+
+```bash
+npm i -g vercel
+```
+
+To use the Flags Explorer, you need to connect your local project to your Vercel project:
 
 ```bash
 vercel link
@@ -44,11 +56,23 @@ vercel env pull
 ### Step 3: Install dependencies
 
 ```bash
-npm install
+pnpm install
+```
+
+or
+
+```bash
+npm install --legacy-peer-deps
 ```
 
 ### Step 4: Run the project
 
+```bash
+pnpm run dev
+```
+
+or
+
 ```bash
 npm run dev
 ```