Add operator documentation

[AMecea]

Create a docs section where to add a starting guide and more:

• How to setup a cluster and fine tuning (detail cluster secret data)
• Detail each cluster param
• How to connect to a cluster (created services)
• How set recurrent backups
• How to migrate to from old cluster to mysql-operator

Also let's add here in comments what needs documentation, maybe I missed something.

[chenmin1992]

really need a document

[AMecea]

Hi @chenmin1992, I will add some doc asap, but until then you can check this page for some basic documentation.

[chenmin1992]

thank you @AMecea, inform me when the doc ready please.

[christianhuening]

would love some more docs too

[AMecea]

addressed by #215 and the documentation can be read at https://www.presslabs.com/code/mysqloperator/getting-started/