Document whether parameters allow nil
Merge pull request #51 from github/jspahrsummers-patch-1

Document whether parameters allow nil
Clarify when strong properties should be used

It wasn't clear to me that the conventions were recommending only using `strong` for mutable properties, so I made it more explicit.
Clarify that we're talking about mutable objects, not readwrite properties
Also mention that strong must be used for non-copyable objects
Minor proofing
Merge pull request #52 from github/aroben-patch-1

Clarify when strong properties should be used
Shit becomes stuff (address #53)
Merge pull request #54 from stanislaw/master

Shit becomes stuff (address #53)
instancetype in initializer instead of id

In the declaration conventions, one of the rule says "Constructors should generally return instancetype rather than id" so perhaps the example of an initializer above should use "instancetype" instead of "id"
Merge pull request #56 from michaelchum/master

instancetype in constructor instead of id for consistency
Merge remote-tracking branch 'upstream/master' into upstream-changes

Conflicts:
	README.md