Add Contributing.md

Contributing.md

@@ -0,0 +1,13 @@
+# Contributing
+
+Thank you for contributing to pyo3!
+
+Here are some things you should check for submitting your pull request:
+
+ - Run `cargo fmt` (This is checked by travis ci)
+ - Run `cargo clippy` and check there are no hard errors (There are a bunch of existing warnings; This is also checked by travis)
+ - If applicable, add an entry in the changelog.
+ - If applicable, add documentation to all new items and extend the guide.
+ - If applicable, add tests for all new or fixed functions
+
+You might want to run `tox` (`pip install tox`) locally to check compatibility with all supported python versions. If you're using linux or mac you might find the Makefile helpful for testing.

Makefile

@@ -1,4 +1,4 @@
-.PHONY: test publish
+.PHONY: test test_py3 publish
 
 test:
 	cargo test