Add README to full media export

[noah-schechter]

The README.txt should contain the following copy:

This folder contains a comprehensive copy of the project "[INSERT PROJECT]" and contains several different types of files and folders. This folder is organized as follows:

• project.json includes general information about the project, including its name, description, and code; its attributes, their descriptions, types, and values; and whether or not the project is archived.
• Each folder contains a single incident's data, which contains a metadata.json file, an updates.json file, and folders for each piece of source material.
• The incident-level metadata.json file contains information about the incident's source material, including each file's hashes.
• The incident-level updates.json file is a log of each update made to an incident, including who changed what data at what time.
• Each piece of source material has a folder which contains visual media and a metadata.json file.
• The source material-level metadata.json file contains information about the source material, including its hashes and source URL.

(Ideally we'd include an ASCII directory tree because this file structure is difficult to communicate without some visual hierarchy, but because it's also a good idea to use plain text, I think this is the best we can do.)