Feature: describe a collection of files with a single metadata document

[davemfish]

Similar to how we can describe a collection that is contained within a zip, it could be useful to describe a collection that is not zipped.

Should this describe all files within a directory? Or a list of files passed by the user? Probably makes sense to handle both cases. For example,

describe_collection(directory='directory', target_yml_filepath='readme.yml')

or

describe_collection(file_list=list_of_filepaths, target_yml_filepath='readme.yml')