Co-authored-by: daxpedda <daxpedda@gmail.com>

The latest rust nightlies and beta (at stable version 1.55) include a
change from

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

to removing `feature(external_doc)` and also changing the syntax of
the second line to

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

However.  `include_str!` is stable currently, but the syntax of `doc =
` is expressly disallowed.  This gives me four options:

 1. Don't build documentation (bad)
 2. Copy-pasta README.md into src/lib.rs (also bad)
 3. Support only beta/nightly but not stable (completely untennable
    for a crate with ~14 million downloads)
 4. Support only stable but not beta/nightly (also untennable)

Further, waiting for this to be "fixed" by its inclusion in stable
Rust in about a week means that our MSRV increases from 1.41 to 1.56,
with no changes to actual code (other than how to build documentation)
at all, which seems quite unfriendly to downstream dependents who are
pinning their rust versions for whatever reason.

So, sadly, it seems the most friendly fix is to copy-pasta our
README.md into the codebase.

(cf. rust-lang/rust#44732,
      rust-lang/rust#82539)

…into develop