30 minute intro: make note of stack vs heap

[steveklabnik]

from here: http://www.reddit.com/r/programming/comments/29h1wf/a_30minute_introduction_to_rust/cikxgo4

It's either 'no systems background required' or 'make sure to adequately explain some systems stuff ,or at least reference other material.'

[steveklabnik]

In the guide, we did this with a link to StackOverflow.

[steveklabnik]

/cc #16641

[steveklabnik]

The intro was just re-done, so closing.