Additional info for docker binary

I found that certain docker installations do not handle binding to the source directory quite right. In my case it was `boot2docker` but I don't think it is specific to it. Updating the docs based on help from `backjlack` and `tibor` in IRC.
Punctuation and syntax isn't right. Suggest:

If you are on a non-Linux platform, e.g., OSX, you'll want to run `make cross` or `make BINDDIR=. cross`.