Explain bundle conventions in the contributing guide

[carolynvs]

We have some unwritten expectations/requirements for example bundles that should be explained in our CONTRIBUTING.md file:

1. name, version and registry are required. Do not use reference.
2. The bundle README.md should provide enough information that someone can copy/paste their way to victory and installing the example bundle. So include any configuration/setup such as required credentials, parameters, or infrastructure (like a cluster). It's okay to link to a doc that explains how to do that stuff, you don't need to repeat it. Also include a "Try it out" section that gives them the install command with all required parameters/flags included.
3. If the bundle provisions any resources, include a "Cleanup" section that walks them through uninstalling the bundle cleanly so that they aren't billed for any lingering resources.