Document easy way and hard way to run Gemini

[bzz]

After #95, update the README structure, add

• "Easy Way" a quickstart for Gemini \w containers
• "Hard Way" as discussed in Making gemini easier to run #84, a separate file under /docs/, with description on how to run it on Apache Spark cluster \w Scylla/Cassandra with all the details
• Apply suggestions from https://github.com/src-d/guide/blob/master/engineering/documentation.md

This change can bee small - it does not need to be "complete" in documentation coverage or full conformance \w doc guide from above, but it has to setup the right "structure" of our user-facing documentation, that will be improved later.

[bzz]

As discussed in #119 (comment) - it would include moving information on running individual dependencies form ./scripts/ to the developer documentation and deleting the scripts

[se7entyse7en]

Sorry I've misclosed it 😅.