Readme should say why this is useful

[cool-RR]

It'll be nice if the readme could answer the question "Why would one want a mutable sequence that can be frozen", i.e. why is this module useful?

[webknjaz]

@mjpieters do you think you could write something up?

[hannes-ucsc]

Is it to a avoid the copy that tuple(somelist) would entail?

[hannes-ucsc]

Also, could freeze() be made to return self and its return type be declared in the stub as Sequence[T]?