Add example of a [[...]] section to src/doc/manifest.md

[casey]

Currentlly, http://doc.crates.io/manifest.html mentions [[...]] style sections but doesn't include an example of one.

I would suggest adding an example of a simple [[bin]] item, so that users don't need to refer to the TOML spec to understand how to use them.

For example:

[[bin]]
name = "my-cool-binary"
path = "src/my-cool-binary.rs"

[panicbit]

The manifest already gives an example using a [lib] section and also mentions that it works the same for the other section types. IMO that should be enough, since the only difference is an additional pair of brackets.

[casey]

Well, just speaking for myself, it wasn't at all obvious to me how to do it from the current description.

[panicbit]

Yeah, a few people on IRC (me included) didn't realise this, so maybe a bit of paraphrasing is needed.

[alexcrichton]

Seems reasonable to me!