Fix README.MD

[Saliva520319]

In apistar.yml, schema.yaml should be schema.yml correspond to up words.

[marcorichetta]

Suggested on #630

[arvinsim]

Honestly the docs are very confusing. It says that I should use apistar validate but I get a Error: Missing argument "schema". error.

Then I tried apistar validate schema.yaml and I got an assertion error. Some parts of the docs says that I need to use schema.json.

So which is it? schema.yml, schema.yaml or schema.json?