Document what unsafety means in rust

[brson]

This is always under debate. Here are some things that are currently considered unsafe:

• Anything that can cause segfaults, invalid reads (memory unsafety)
• Anything that can create invalid utf8 strings
• Anything that can invoke undefined behavior (ptr::offset)
• Data races
• Deadlocks (in extra::arc)

Things that are not typically considered unsafe:

• Deadlocks

[thestinger]

We can hash this out in a wiki page before turning it into official documentation:

https://github.com/mozilla/rust/wiki/Unsafety-draft

[pnkfelix]

Nominating for milestone 1: Well defined.