Add LICENSE file to cargo package

This is required by MIT terms "The above copyright notice and this
permission notice shall be included in all copies or substantial
portions of the Software."

Some tools will check for the file to be present before vendoring.

Cargo.toml

@@ -9,7 +9,7 @@ repository = "https://github.com/pseitz/lz4_flex"
 readme = "README.md"
 license = "MIT"
 version = "0.11.1"
-include = ["src/*.rs", "src/frame/**/*", "src/block/**/*", "README.md"]
+include = ["src/*.rs", "src/frame/**/*", "src/block/**/*", "README.md", "LICENSE"]
 
 [package.metadata.docs.rs]
 all-features = true