add contributing guide

CONTRIBUTING.md

@@ -0,0 +1,16 @@
+Please ensure that all tests and linter checks passed on your branch before creating a PR which modifies some source files.
+
+To run tests:
+
+```sh
+cargo test -- --skip src/lib.rs
+```
+
+`--skip` is necessary since `cargo test` tries to run code blocks in [README file](./README.md).
+
+To run linters:
+
+```sh
+cargo clippy --all-features --examples
+cargo fmt -- --check
+```