Adds environment list and info about README

Added blurb about self-sufficient README for deploys
Added running environment minimal list
Adds musts about sanitization and Gem versions

Added requirement to add versions in Gemfile is some specific version is
needed.
Added requirement to sanitize config inputs.
Added note on logging

A logger should be used instead of puts.
Adds note about migration checking

Added migration check example task.
This check helps achieving idempotency in deployment scripts &
playbooks.
Merge branch 'devel'

* devel:
  Adds note about migration checking
  Added note on logging
  Adds musts about sanitization and Gem versions
  Adds environment list and info about README

Conflicts:
	README.md