248: Update README w/ Clang + Sodium version r=Dylan-DPC a=nvesely

bindgen recommends Clang >= 3.9 for C++. Earlier versions of libsodium seem not to be building (see #244 and #245), however, the requirement to manually install the latest version if it's not available on your distro is a temporary fix until #234 is merged.

Co-authored-by: Noah Vesely <fowlslegs@riseup.net>