Add schema , renderMark and renderNode default functions as exports

[Ognian]

You have already added the renderMark and renderNode functions to the readme, but I would suggest to add them as exports too.
I would also suggest to add the schema function used for normalization as an export. In the slate-36 branch of rich-markdown-editor
https://github.com/outline/rich-markdown-editor/blob/slate-36/src/schema.js . Whith this Everything for a quick start for a markdown Editor with slate would be in this repository and it would be easier to follow the versions...
What do you think?
Thanks
Ognian