Add minimal README.md files to published crates.

This will put descriptions on the packages' crates.io pages.

lib/wasm/Cargo.toml

@@ -6,6 +6,7 @@ publish = false
 description = "Translator from WebAssembly to Cretonne IL"
 repository = "https://github.com/stoklund/cretonne"
 license = "Apache-2.0"
+readme = "README.md"
 
 [lib]
 name = "cton_wasm"

lib/wasm/README.md

@@ -0,0 +1,3 @@
+This crate performs the translation from a wasm module in binary format to the
+in-memory representation of the [Cretonne](https://crates.io/crates/cretonne)
+IL.