Improve README with usage section and more descriptive section

Author: Urgau

README.md

@@ -1,2 +1,32 @@
 # rustc-stable-hash
-A stable hashing algorithm used by rustc: cross-platform, deterministic, not secure
+
+[![crates.io](https://img.shields.io/crates/v/rustc-stable-hash.svg)](https://crates.io/crates/rustc-stable-hash)
+[![Documentation](https://docs.rs/rustc-stable-hash/badge.svg)](https://docs.rs/rustc-stable-hash)
+
+A stable hashing algorithm used by `rustc`: cross-platform, deterministic, not secure.
+
+This crate provides facilities with the `StableHasher` structure to create stable hashers over *unstable* hashers by abstracting over them the handling of endian-ness and the target `usize`/`isize` bit size difference.
+
+Currently, this crate provides it's own implementation of 128-bit `SipHasher`: `SipHasher128`; with `StableSipHasher128` for the stable variant. 
+
+## Usage
+
+```rust
+use rustc_stable_hash::hashers::{StableSipHasher128, SipHasher128Hash};
+use rustc_stable_hash::FromStableHash;
+use std::hash::Hasher;
+
+struct Hash128([u64; 2]);
+impl FromStableHash for Hash128 {
+    type Hash = SipHasher128Hash;
+
+    fn from(SipHasher128Hash(hash): SipHasher128Hash) -> Hash128 {
+        Hash128(hash)
+    }
+}
+
+let mut hasher = StableSipHasher128::new();
+hasher.write_usize(0xFA);
+
+let hash: Hash128 = hasher.finish();
+```