Add usage example in README.

Author: SimonSapin

README.md

@@ -5,3 +5,74 @@
 [Documentation](https://docs.rs/string_cache/)
 
 A string interning library for Rust, developed as part of the [Servo](https://github.com/servo/servo) project.
+
+## Simple usage
+
+In `Cargo.toml`:
+
+```toml
+[dependencies]
+string_cache = "0.3"
+```
+
+In `lib.rs`:
+
+```rust
+extern crate string_cache;
+use string_cache::DefaultAtom as Atom;
+```
+
+## With static atoms
+
+In `Cargo.toml`:
+
+```toml
+[package]
+build = "build.rs"
+
+[dependencies]
+string_cache = "0.3"
+
+[build-dependencies]
+string_cache_codegen = "0.3"
+```
+
+In `build.rs`:
+
+```rust
+extern crate string_cache_codegen;
+
+use std::env;
+use std::path::Path;
+
+fn main() {
+    string_cache_codegen::AtomType::new("foo::FooAtom", "foo_atom!")
+        .atoms(&["foo", "bar"])
+        .write_to_file(&Path::new(&env::var("OUT_DIR").unwrap()).join("foo_atom.rs"))
+        .unwrap()
+}
+```
+
+In `lib.rs`:
+
+```rust
+extern crate string_cache;
+
+mod foo {
+    include!(concat!(env!("OUT_DIR"), "/foo_atom.rs"));
+}
+```
+
+The generated code will define a `FooAtom` type and a `foo_atom!` macro.
+The macro can be used in expression or patterns, with strings listed in `build.rs`.
+For example:
+
+```rust
+fn compute_something(input: &foo::FooAtom) -> u32 {
+    match *input {
+        foo_atom!("foo") => 1,
+        foo_atom!("bar") => 2,
+        _ => 3,
+    }
+}
+```