Better getting started help text

[ejholmes]

Right now, If I cd to an empty directory, and execute walk, this is what I see:

dir $ walk
dir $

No output. walk is doing the right thing, since the intended behavior of a dependency in a directory without a Walkfile is to assume that the dependency is a static file, or a noop dep.

However, in the above case, where I'm executing a root target that has no Walkfile, it would be nice to output some helpful message on stderr:

dir $ walk
WARNING: No Walkfile found for target "all"
See https://ejholmes.github.io/walk#getting-started