message: doc(cache) Improve the `README.md` with an example.
diff: # Wasmer Cache
  -The `wasmer-cache` crate allows easily caching WebAssembly
  -modules in your system.
  +The `wasmer-cache` crate allows to cache WebAssembly modules (of kind
  +`wasmer::Module`) in your system, so that next uses of the module does
  +imply a compilation time.
  +
  +## Usage
  +
  +The `Cache` trait represents a generic cache for storing and loading
  +compiled WebAssembly modules. The `FileSystemCache` type implements
  +`Cache` to store cache on the file system.
  +
  +```rust
  +use wasmer::{DeserializeError, Module, SerializeError};
  +use wasmer_cache::{Cache, FileSystemCache, Hash};
  +
  +fn store_module(module: &Module, key: &[u8]) -> Result<(), SerializeError> {
  + // Create a new file system cache.
  + let mut fs_cache = FileSystemCache::new("some/directory/goes/here")?;
  +
  + // Compute a key for a given WebAssembly binary
  + let hash = Hash::generate(key);
  +
  + // Store a module into the cache given a key
  + fs_cache.store(hash, module.clone())?;
  +
  + Ok(())
  +}
  +```
  