Update README.md in both REST and GraphQL branches

[TheYuriG]

There hasn't been any updates to either branch README.md files in a while.

While both branches perform about exactly the same operations, the REST API heavily uses the power of Express.js to handle routing, authentication, data validation, and so on, while the GraphQL handles everything with resolvers.

It's important to state that both branches work separately and they have their key differences.

When updating the README.md for both branches, make sure to have a link to the corresponding branch of the frontend and explicitly state that only the REST branch has access to webhooks.