add a gitignore
update exceptions doc and policy as discussed in #16.
merged content of team faq into practice
link to guidance doc in readme
whoops, remove dupe data
porting over design limitation to confidentiality section, cc @seanherron
Reorganize and reword to match our consensus from this week's meeting.

We are instead applying our narrow exceptions to working in public
from day 1, not just eventual release of source code.
Merge pull request #18 from 18F/harmonize

Clarify policy on working in the open, rewrite/merge team guidance