New evolve docs: sharing guide

[Greg Ward]

On 15 June 2014, timeless at gmail.com said:
> > abort: cannot amend public changesets
> 
> That's a poor error message...
> I'd expect to be explicitly informed that my commit is frozen...‎

Out of scope right now. I'm just trying to document Mercurial +
evolve.

(Also, the error message makes sense to me. "cannot amend" is pretty
clear. It's *not* immediately obvious that "public" == "immutable",
but that's why I'm spending some time in the evolve docs to explain
what phases mean... namely, that "public" == "immutable". But the
error message "cannot amend public changesets" pretty much tells you
that "public" == "immutable".)

       Greg
-- 
Greg Ward                            http://www.gerg.ca
<greg at gerg.ca>                       @gergdotca