Expand Readme to include block types

[holdenmad]

As it took me a little longer than normal to find the right block types to render things correctly, I would suggest adding this to the Readme: https://github.com/contentful/rich-text/blob/master/packages/rich-text-types/src/blocks.ts

Although there is a link to the rich-text types, I don't see any reason why you shouldn't link directly to the blocks.ts which contains the exact syntax for the types.