Add documentation about executable with native dependencies in same project

[tionis]

This was mainly discussed in #71 especially the last few comments. As @sogaiu mentioned a slight rewording and some examples that use (declare-executable :deps [(dep :static)] would be useful.

[sogaiu]

In case more comments show up in #71, I'll add a link here indicating approximately where our discussion was :)

[tionis]

Thanks, I forgot that you can do that