Document cargo features in readme

[rukai]

Users who use this crate as library will very likely want the cli feature disabled and its also not clear to me what the nameattr feature is for even after looking at the code.
So a section in the readme or maybe the root of the crate docs that lists what every feature is and which are enabled by default would be really valuable.

[jonhoo]

I completely agree! I really wish there was a standard way to document features in Cargo/the Rust ecosystem. There's some discussion in rust-lang/api-guidelines#43, but no hard guidance as of yet. @itamarst maybe you could take a stab at this after you land #294?