Apply README tweaks.
Apply README tweaks.

To make consistent with libbitcoin & libbitcoin-explorer READMEs.
Change OSX => macOS.
Reword Configuration Options heading.
Add Build Option reference to 3 install steps.
Simplify Homebrew install step, removing description of dependencies.

RojavaCrypto/libbitcoin-server,RojavaCrypto/libbitcoin-server,RojavaCrypto/libbitcoin-server