Guide: if

Adding the section about if to the guide. I hope this won't conflict with https://github.com/rust-lang/rust/pull/15229 , but we'll just have to see, I guess.
consider adding a comment saying that this is erroneous code.

i get this from the current `rustc`:
```rust
let x = (let y = 5); // error: found `let` in ident position
```
but I guess if we put that error output into the example, we'll need one more sentence explaining "The compiler is telling us here that it was expecting to see the beginning of an expression, and a `let` can only begin a statement, not an expression."