Document rustfmt on nightly-rustc

The recursion_limit attribute avoids the following error:

```
error[E0275]: overflow evaluating the requirement `std::ptr::Unique<rustc_ast::Pat>: std::marker::Send`
  |
  = help: consider adding a `#![recursion_limit="256"]` attribute to your crate (`rustfmt_nightly`)
```

1 files changed, 1 insertions, 0 deletions

diff --git a/src/lib.rs b/src/lib.rs
index cde5d390cf2..ce8a45eea65 100644
--- a/src/lib.rs
+++ b/src/lib.rs
@@ -1,6 +1,7 @@
 #![feature(rustc_private)]
 #![deny(rust_2018_idioms)]
 #![warn(unreachable_pub)]
+#![recursion_limit = "256"]
 
 #[macro_use]
 extern crate derive_new;