add CONTRIBUTING guide (#28)

Author: danieleades

CONTRIBUTING.md

@@ -0,0 +1,19 @@
+# Contributing
+
+## Run the tests
+
+        cargo test
+
+## Run the bench tests
+
+        cargo bench
+
+## Run the fuzz tests
+
+(requires [cargo-fuzz](https://github.com/rust-fuzz/cargo-fuzz))
+
+        cargo fuzz run fuzz_target_1
+
+## Run the examples
+
+        cargo run --example=${EXAMPLE}