Add featured documentation

[mtth]

Suggestion from https://github.com/mtth/avsc/issues/255.

[RandomFractals]

hey Matt, whenever you get to it, I really like how Apache Arrow dev crew set it up to feature projects using their tech.:

https://arrow.apache.org/powered_by/

something along those lines in md docs with links to other tools and cool libs using your avro lib is what I thought would be nice for devs to explore.

& thanks for this great Avro parsing/loading library. It's awesome!