Prefer implicit getters on computed properties

Proposed by @robrix in https://github.com/ReactiveCocoa/ReactiveCocoa/pull/1382#discussion_r14852078.

/cc @github/mac
Update README.md
Update README.md
Add subscripting to the guideline as well
Merge pull request #1 from github/jspahrsummers-patch-1

Prefer implicit getters on computed properties