Add docs about why & how to install the ffi.h dependency

as suggested by @TimDaub in https://github.com/bigchaindb/bigchaindb/issues/694#issuecomment-260358428

They could follow the same pattern as https://docs.bigchaindb.com/projects/server/en/latest/appendices/install-os-level-deps.html

Start by reading the referenced appendices/install-os-level-deps.html page to follow its documentation pattern, then locate the corresponding documentation entry point in cryptoconditions. Document why ffi.h is needed and how to install it; the issue is complete when both points are covered clearly.

Written by the indexing model from the issue text.