Remove deprecated cfg_attr from README
Remove deprecated cfg_attr from README

`tool_attributes` are stable since 1.30. The old `cfg_attr(rustfmt, rustfmt_skip)` attributes aren't necessary anymore and everyone should switch to `#[rustfmt::skip]` sooner or later.

There is also a Clippy lint in the making for this: rust-lang-nursery/rust-clippy#3123
rust-lang/rustfmt,graydon/rust,SingingTree/rustfmt,graydon/rust,graydon/rust,graydon/rust,graydon/rust,aidancully/rust,graydon/rust,aidancully/rust,aidancully/rust,rust-lang/rustfmt,SingingTree/rustfmt,aidancully/rust,aidancully/rust,aidancully/rust