[Angular-Apollo-Tailwind] Update README with the correct setup

[jesus4497]

Acceptance

• README mentions using npm but yarn should be preferred if we are using yarn.lock file.
• There should be a mention to run yarn generate.
• .env.example doesn't provide a proper initial config. The following setup worked for me:

GITHUB_GRAPHQL_URL=https://api.github.com/graphql
OAUTH_API_URL=https://api.starter.dev
CLIENT_REDIRECT_URL=http://localhost:4200/redirect
PRODUCTION=false