Add CF-specific README

Author: kornelski

README.md

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+# Cloudflare fork of Zlib
+
+This is a fork of zlib with performance improvements developed for use at Cloudflare.
+
+This implementation only supports x86-64 with SSE 4.2+ and aarch64 with NEON & CRC. 32-bit CPUs are not supported at all.
+
+The API and ABI are compatible with the original zlib.
+
+This library is also available as a [Rust crate](https://lib.rs/crates/cloudflare-zlib).