Documenting how the filter attribute of py_layer works

[guillaumep]

The README file gives examples on how to use py_layer and its filter attribute:

https://github.com/bazelbuild/rules_docker#py_image-fine-layering

However it does not say how exactly the filter works. I had to check the source code to confirm that the filtering was done using startswith:

https://github.com/bazelbuild/rules_docker/blob/1b04e4456c5c4a8109a2b673f2734cb5ce39d019/lang/image.bzl#L320

Could I suggest that the documentation be updated to mention this important piece of information?