Document support for `lz4framed`

[jeffwidman]

#1395 added support for lz4framed, but didn't document it.

I started to add it, and realized I really don't understand the differences between these two...

@everpcpc can you clarify what you found compelling about lz4framed over lz4? At a quick glance, both appear to be just bindings to C implementations, so it's not like one is easy to debug (pure python) and the other is fast (C based)...

[everpcpc]

Actually it's just an alternative. Because lz4 has no stable support for framed format before 1.0.0.
Besides, they are both listed in http://lz4.github.io/lz4/.