Principia formula guidelines

[Clint Byrum]

Excerpts from Gustavo Niemeyer's message of Mon Jun 06 15:30:42 -0700 2011:
> > I've created a wiki page detailing what I think are the important points
> > to start enabling formula contributors, as I've had a few "hey can I
> > get my formula into Principia" requests from people so far.
> >
> > https://ensemble.ubuntu.com/Principia
> 
> This looks great!
> 
> It might be good to have an extra step before 5 there for creating the
> formula directory, just because it may not be immediately obvious what
> "your formula root" is yet given the context.

I added a clarification that the place where 'getall' is pointed at is
the formula repository.

> 
> Also, a pair of links into the documentation might be helpful to
> provide hints of what the metadata is, how to create the hooks, etc.
> 

I linked to the tutorial at the top.

> Re. Hooks should be idempotent, this is going to make people wonder.
> We may need some secondary explanation about why it is there and how
> to achieve it.

Added a brief explanation of why idempotency of hooks is important.

> 
> Great stuff, thanks!
> 

:)