README: remove old requirement for Rust 2018 edition use

From now on, the `Cargo.toml` is taken into account when triggering
formatting using `cargo fmt`.

It is considered editor's duty to pass the proper `--edition` argument
for `rustfmt` if it is being called manually.

Refs: rust-lang#3104.
Refs: rust-lang#3129.

Signed-off-by: Otavio Salvador <[email protected]>

Author: otavio

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