Add docs how worker uses crispy-cli

[vitaliimelnychuk]

As we have workers that communicate with crispy-api for the following reasons:

• API sends a request when we need to run the migration
• Worker sends to API what the current status of migration

We need to describe it in details what kind of requests we need from API, and how crispy-cli will be used here.
Initially, the idea was to have a lot different workers to run migrations in different servers and have runners installed there.
But we need crispy-cli as migration runner and migrations schema parser.
We would like to run particular crispy-cli command when we receive some command from API.

As a result here, should be:

• detailed schema how crispy-cli whould be installed here
• the main purpose of service
• what we need from API
• what we give to API