Update readme.
filepath: Prevent FilePath.child() from returning itself.

Well, not exactly itself, but an equivalent and nearly-identical path, at
least.

The backstory's in Twisted #6728; this commit could probably be ported without
too much trouble.
generic: genericWalk() should only check whether to recurse on dirs.

Also add a test and clean up the docstring.

See Twisted #5123 for more information; this could be ported easily.
filepath: Random small patchups to docstrings.