Merge pull request rust-lang#3133 from otavio/rework-readme

README: remove old requirement for Rust 2018 edition use

Author: nrc

README.md

@@ -36,12 +36,6 @@ To run:
 cargo +nightly fmt
 ```
 
-To format code that requires edition 2018, create a `rustfmt.toml` [configuration](#configuring-rustfmt) file containing:
-
-```toml
-edition = "2018"
-```
-
 ## Limitations
 
 Rustfmt tries to work on as much Rust code as possible, sometimes, the code