updated README.md for new API.
updated README.md for new API.

nox/rust-encoding,servo/rust-encoding,servo/rust-encoding,nox/rust-encoding,lifthrasiir/rust-encoding,jedisct1/rust-encoding,tempbottle/rust-encoding,lifthrasiir/rust-encoding,lifthrasiir/rust-encoding,tempbottle/rust-encoding,nox/rust-encoding,tempbottle/rust-encoding,jedisct1/rust-encoding