Compilation failure with rust-nightly because of `external_doc` feature

[camelop]

subtle-ng/src/lib.rs

Line 12 in 73b8a16

#![cfg_attr(feature = "nightly", feature(external_doc))]

The current version (2.4.1) fails to compile with rustc 1.54.0-nightly (35fff69d0 2021-06-06) because the "external_doc" feature has been removed. For more information, see this PR.

[hdevalence]

Not sure what the best way to fix this is.

Switching to

#![cfg_attr(feature = "nightly", doc = include_str!("../README.md"))]

doesn't work on stable, because the right hand side of the cfg_attr has unstable syntax, even though it's not taken.

This is supposed to be stable in 1.54, but I think that doesn't land for three weeks or so.