Document "Default object lifetime bounds"

[bluss]

"Default object lifetime bounds" rust-lang/rfcs/pull/1156

This should be mentioned in the language reference.

[steveklabnik]

The reference has been moved out to its own repo, but @frewsxcv says he's gonna look at it and i will assist as needed.

[brson]

Ping @frewsxcv. How's this going?

[frewsxcv]

I really do want to work on this but it's pretty far down my work queue right now and I haven't started on it.

[steveklabnik]

Re-assigning to @QuietMisdreavus ❤️