Clearer info for how to contribute to an existing gem

[ronen]

So that willing contributors won't get stymied. Two things are needed:

• Update the boilerplate "things you need to know" to make clear that you need to install schema_dev, and that you should read the schema_dev README
• The README should start with instructions for contributors to existing gems, with step-by-step instructions for what to do. Later on it can info on how to create a new gem, and other details.