rust: zerocopy: add `README.md`

rust: zerocopy: add `README.md`

Originally, when the Rust upstream `alloc` standard library crate was
vendored in commit 057b8d257107 ("rust: adapt `alloc` crate to the
kernel"), a `README.md` file was added to explain the provenance and
licensing of the source files.

Thus do the same for the `zerocopy` crate.

Cc: Joshua Liebow-Feeser <joshlf@google.com>
Cc: Jack Wrenn <jswrenn@google.com>
Link: https://patch.msgid.link/20260608141439.182634-12-ojeda@kernel.org
Signed-off-by: Miguel Ojeda <ojeda@kernel.org>