Update README.md

[shutton]

The README is a bit out of date, and will also need to be updated when clamav-async-rs is published.

• Change references to clamav-rs to clamav-async, as clamav-rs will be deprecated.
• macOS instructions should reference using homebrew both for installing OpenSSL, but also ClamAV
• Remove references to libclamav-sys.
• Specify the associated version of libclamav
• Clarify whether clamav-dev Unix packages are required. Only libclamav should be required, which is a runtime library.

[shutton]

@zaddach & @micahsnyder: your thoughts would be welcome, particularly on the Windows bits.