doc: rudimentary developer docs for state
doc: rudimentary developer docs for state

This has been dashed off without deep thought; I'm sure there are bits
missing. If you, dear reader, are not especially familiar with the nuts
and bolts of state, I would particularly appreciate your input.

R=rog, jameinel, dimitern
CC=
https://codereview.appspot.com/7631046

bogdanteleaga/juju,dooferlad/juju,marcoceppi/juju,mhilton/juju,perrito666/juju,mjs/juju,frankban/juju,reedobrien/juju,sinzui/juju,mattyw/juju,howbazaar/juju,marcmolla/juju,bz2/juju,voidspace/juju,gabriel-samfira/juju,reedobrien/juju,frankban/juju,fwereade/juju,mhilton/juju,rogpeppe/juju,kat-co/juju,waigani/juju,mjs/juju,makyo/juju,anastasiamac/juju,frankban/juju-tmp,dooferlad/juju,mhilton/juju-juju,marcmolla/juju,mattyw/juju,cloud-green/juju,marcmolla/juju,AdamIsrael/juju,cloud-green/juju,sinzui/juju,kat-co/juju,ericsnowcurrently/juju,reedobrien/juju,frobware/juju,fwereade/juju,cmars/juju,waigani/juju,cmars/juju,cloud-green/juju,mhilton/juju-juju,Altoros/juju-cloudsigma,AdamIsrael/juju,voidspace/juju,bogdanteleaga/juju,kat-co/juju,perrito666/juju,macgreagoir/juju,marcoceppi/juju,reedobrien/store,frobware/juju,AdamIsrael/juju,mikemccracken/juju,marcmolla/juju,tych0/juju,tasdomas/juju,rogpeppe/juju,dooferlad/juju,tsakas/juju,Altoros/juju-vmware,gabriel-samfira/juju,alesstimec/juju,mjs/juju,alesstimec/juju,perrito666/juju,bz2/juju,frankban/juju,axw/juju,gabriel-samfira/juju,bz2/juju,Altoros/juju-vmware,kat-co/juju,fwereade/juju,makyo/juju,cherylj/juju,axw/juju,mikemccracken/juju,rogpeppe/juju,bz2/juju,frankban/juju-tmp,davecheney/juju,jrwren/juju,dimitern/juju,bogdanteleaga/juju,thomnico/juju,sinzui/juju,tych0/juju,howbazaar/juju,cherylj/juju,ericsnowcurrently/juju,cherylj/juju,waigani/juju,dimitern/juju,wwitzel3/juju,howbazaar/juju,mhilton/juju,reedobrien/juju,mhilton/juju,thomnico/juju,perrito666/juju,mikemccracken/juju,mjs/juju,makyo/juju,sinzui/juju,reedobrien/juju,axw/juju,davecheney/juju,ericsnowcurrently/juju,wwitzel3/juju,alesstimec/juju,mhilton/juju-juju,tasdomas/juju,bac/juju,frankban/juju-tmp,cloud-green/juju,alesstimec/juju,macgreagoir/juju,marcmolla/juju,thomnico/juju,AdamIsrael/juju,mikemccracken/juju,howbazaar/juju,cmars/juju,ericsnowcurrently/juju,frankban/juju-tmp,gabriel-samfira/juju,thomnico/juju,tych0/juju,davecheney/juju,ericsnowcurrently/juju,Altoros/juju-cloudsigma,mattyw/juju,anastasiamac/juju,jrwren/juju,alesstimec/juju,tasdomas/juju,macgreagoir/juju,perrito666/juju,cherylj/juju,davecheney/juju,macgreagoir/juju,bac/juju,mjs/juju,mikemccracken/juju,wwitzel3/juju,Altoros/juju-cloudsigma,tsakas/juju,mjs/juju,jrwren/juju,Altoros/juju-cloudsigma,anastasiamac/juju,dimitern/juju,bogdanteleaga/juju,dooferlad/juju,marcoceppi/juju,bac/juju,macgreagoir/juju,mhilton/juju-juju,bac/juju,axw/juju,voidspace/juju,dimitern/juju,tsakas/juju,cmars/juju,marcoceppi/juju,mattyw/juju,anastasiamac/juju,kat-co/juju,tsakas/juju,tych0/juju,tasdomas/juju,voidspace/juju,mjs/juju,frankban/juju,makyo/juju,bac/juju,wwitzel3/juju,dimitern/juju,fwereade/juju,howbazaar/juju,frankban/juju,waigani/juju,bz2/juju,Altoros/juju-vmware,gabriel-samfira/juju,anastasiamac/juju,axw/juju,Altoros/juju-vmware,frobware/juju,dooferlad/juju