Migrate more README over to the docs

docs/contributing.md

@@ -0,0 +1,21 @@
+Contributing
+============
+
+Wheel-building is pretty complex. I expect users to find many edge-cases - please help the rest of the community out by documenting these, adding features to support them, and reporting bugs.
+
+I plan to be pretty liberal in accepting pull requests, as long as they align with the design goals below.
+
+`cibuildwheel` is indie open source. I'm not paid to work on this.
+
+Design Goals
+------------
+
+- `cibuildwheel` should wrap the complexity of wheel building.
+- The user interface to `cibuildwheel` is the build script (e.g. `.travis.yml`). Feature additions should not increase the complexity of this script.
+- Options should be environment variables (these lend themselves better to YML config files). They should be prefixed with `CIBW_`.
+- Options should be generalise to all platforms. If platform-specific options are required, they should be namespaced e.g. `CIBW_TEST_COMMAND_MACOS`
+
+Other notes:
+
+- The platforms are very similar, until they're not. I'd rather have straight-forward code than totally DRY code, so let's keep airy platfrom abstractions to a minimum.
+- I might want to break the options into a shared config file one day, so that config is more easily shared. That has motivated some of the design decisions.