Clarify in the README the directory structure

[nigelmegitt]

    Let me review the tests in the imsc1 folder before re-proposing any changes if required....

"The tests are layered, as is the functionality required in the different versions of the IMSC specification. Tests that apply to all versions are in the IMSC1 directory, ones that apply to functionality introduced in 1.1 would be in the 1.1 directory, likewise with 1.2 etc. So a 1.2 implementation should be tested against the tests in the 1, 1.1 and 1.2 directories."
@nigelmegitt - please can we add this text to the readme for the repo - just to make it explicit.

Originally posted by @btsimonh in #105 (comment)

[palemieux]

Tests that apply to all versions are in the IMSC1 directory,

This is not exactly true for deprecated features, which might ultimately be removed.

[nigelmegitt]

This is not exactly true for deprecated features, which might ultimately be removed.

That's a good point. We should think of a way to indicate those deprecated features.
Perhaps a manifest file for each version of IMSC, that lists all the relevant tests for that version?