Need to document dependency on snappy and native build of util-mmap

[youknowjack]

lsmtree depends on util-compress, which depends on a native snappy build.
It also depends on util-mmap, which by default contains a native linux build.

Need to figure out the best way to document these dependencies and explain how to build/run on different (compatible) platforms.

[andy-yx-chen]

With them, it's not 100% java, right?