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.