Add mdast-flatten-listitem-paragraphs to readme.md

[staltz]

No description provided.

[wooorm]

Thanks for your contributions, @staltz! Awesome things! What are you working on?

[staltz]

Thanks for merging! This other PR explains what I'm working on: remarkjs/react-markdown#199

[wooorm]

@staltz Sweet, interested to see where this goes, and let me know if you have questions re unified/remark/mdast/etc!

Oh and as an aside: stuff that can be passed to unified().use() is typically called remark- (or unified-), instead of mdast-. Maybe you already knew, and it’s not a problem for these little mdast utilities, but if you’d like to wrap them all into one plugin I suggest using remark- as a prefix!

[staltz]

Thanks for the tips!