Update the documentation / readme

[lpuskas]

• add sections / document newly added providers
• document required configuration (env vars)

[martonsereg]

The documentation still doesn't contain anything about how to setup a Vault instance that is able to handle token management, how to start Pipeline and get a valid token, and what is a token signing key and how do I get one.
If there are descriptions about these in other readmes then links are perfectly fine, but the current documentation still falls short of being a proper resource to start an authentication-enabled Telescopes API in a few minutes.

[lpuskas]

Pipeline / Vault setup description:

https://github.com/banzaicloud/pipeline/blob/master/docs/github-app.md