[GraphQL Plugin] README for GraphQL Plugin

[taras]

TODO

• [x] Installation instructions
• [x] How to add GraphQL to GraphiQL plugin?
• [x] How to integrate with Backstage API Docs Plugin?
• [x] How to use built in directives?
• [ ] How to extract the schema?
• [x] How do you extend the schema from packages/backstage/src/plugins/graphql?
• [ ] How to extend schema from a plugin?
• [ ] How to add new directives?
• [ ] How to connect to the GraphQL API from the frontend?
• [ ] How to test the GraphQL API?