Improve the README development setup steps around environment variables (#4034)

* Set the NEXTAUTH_URL in the .env example

* Add a .env step to the README development setup steps.

Co-authored-by: kodiakhq[bot] <49736102+kodiakhq[bot]@users.noreply.github.com>

.env.example

@@ -46,8 +46,7 @@ PGSSLMODE=
 # @see: https://github.com/calendso/calendso/issues/263
 # @see: https://next-auth.js.org/configuration/options#nextauth_url
 # Required for Vercel hosting - set NEXTAUTH_URL to equal your NEXT_PUBLIC_WEBAPP_URL
-# NEXTAUTH_URL='http://localhost:3000'
-NEXTAUTH_URL=
+NEXTAUTH_URL='http://localhost:3000'
 # @see: https://next-auth.js.org/configuration/options#nextauth_secret
 # You can use: `openssl rand -base64 32` to generate one
 NEXTAUTH_SECRET=