Add README.md to package

[fiji-flo]

The missing README.md breaks vendored builds depending on tiktoken-rs.

In any project using tikoken-rs do:

mkdir .cargo
cargo vendor > .cargo/config
cargo build --release

...

error: couldn't read /tmp/my-project-vendored/vendor/tiktoken-rs/src/../README.md: No such file or directory (os error 2)
 --> /tmp/my-project-vendored/vendor/tiktoken-rs/src/lib.rs:1:10
  |
1 | #![doc = include_str!("../README.md")]
  |          ^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  |
  = note: this error originates in the macro `include_str` (in Nightly builds, run with -Z macro-backtrace for more info)

[zurawiki]

Thanks for the patch. Repros locally and can confirm the fix