Added basic doc for HA
Added singleton and voting sections and corrected

I addressed several redaction issues and added sections for
voting and singleton workers as commented by @rogpeppe
Made gramatical corrections suggested by @mjs
Added note on singleton vs concurrent

After talking with @fwereade I added a note explaining that
singletons, as they are documented righ now, are only a reflection
of current status but should be considered as wrong and will be
re-implemented properly, as concurrent workers, in the future.
Corrected two gramatical errors
Merge pull request #30 from perrito666/update_HA_doc

Added basic doc for HA

I created a doc for High Availability based on code analysis and interviewing people from the team that where involved in the design and development of this.
WIP basic commit to get Action Hooks rolling