Guide: ownership

Whew. This section was so important, I saved it for last.

/cc @wycats, @nikomatsakis
Here's how I'd explain it:

1. With any borrow, you can read, and re-lend to others to read.

2. With a mutable borrow, you can also write, and re-lend to others to *write. (But to prevent data races, you temporarily give up your ability to write if you re-lend, and even your ability to read if you re-lend mutably.)

3. You relinquish your borrow when you go out of scope. (Conveniently, your sub-borrowers had to do the same, so they don't prevent you from doing this.)