Add instructions for rustbook build via mirror

[jameslzhu]

Replaces old instructions of compiling rustc from scratch.

Instead of compiling rustc, I've found a mirror of the rustbook code.

Both rustbook and the book compile without problems on my Linux box:
Arch Linux x86_64 (kernel 4.1.3-1-ARCH)
rustc 1.3.0-nightly (4dfe7a16c 2015-07-30)
cargo 0.4.0-nightly (e0a82d6 2015-07-29) (built 2015-07-30)

I'm fairly certain most people will want to avoid bootstrapping the entire rustc compiler if the mirror works just as well.

[Gankra]

I gave these instruction specifically because it didn't work on my mac.

Have you built rustc on your machine before?

[jameslzhu]

Would it be better if I added both sets of instructions to the readme then?

[Gankra]

I'll try to use steve's fork again first...

[Gankra]

Yeah I just get

rustbook::./target/release/rustbook
dyld: Library not loaded: x86_64-apple-darwin/stage2/lib/rustlib/x86_64-apple-darwin/lib/librustdoc-74fa456f.dylib
  Referenced from: /Users/ABeingessner/dev/rustbook/./target/release/rustbook
  Reason: image not found
Trace/BPT trap: 5

As usual. Maybe it's because I do build the compiler or maybe it's because I use multirust. shrug

[jameslzhu]

I'm running multirust as well (multirust 0.0.6 (6b18101d0 2015-04-30 14:40:11 -0700), so I don't think that's the issue.
For now, I'll add your instructions in as a fallback in case people encounter your error.
Sound good?

[Gankra]

👍

[Gankra]

I have a sneaking suspicion it's because you and @steveklabnik use Linux -- but also dynamic linking is just dumb and butts.

[jameslzhu]

Ready to go.

Also, yes, dynamic linking has bitten me way too many times.

[Gankra]

Can you add rustbook specifically to the end of the path? I reckon that's a bit easier to copy-paste as appropriate (and means they don't have to think about it).

[Gankra]

Whoops, didn't notice you did the fix I requested (github doesn't notify about pushes).

Thanks a bunch!