suggested enhancement - document required bit operation support

torch-hdf5 [requires](https://github.com/deepmind/torch-hdf5/blob/master/luasrc/ffi.lua#L3) bit operation support. Although LuaJIT has such support built-in, Lua 5.1 and 5.2 do not and may require explicit installation of [LuaBitOp](http://bitop.luajit.org). It therefore might be a good idea to document this dependency in the installation instructions.

Start with luasrc/ffi.lua, especially the linked dependency reference, then locate the installation instructions. Document that Lua 5.1 and 5.2 require explicit LuaBitOp support while LuaJIT provides bit operations built in; the installation documentation is done when this dependency distinction is clear.

Written by the indexing model from the issue text.