Improve documentation / point to available tutorials

[kosmikus]

I've given quite a few talks and written a few tutorials by now (others might have done the same), yet the package itself only points to the original paper.

We might want to create more documentation / an official tutorial. Or at least have a place where we collect and point to available materials.

I'm not even sure if I'm aware of all, but certainly there are at least:

• my HaskellX Bytes 2017 talk: https://skillsmatter.com/skillscasts/10181-fun-with-sum-and-product-types, https://github.com/kosmikus/sop-haskellx-bytes-2017
• my ZuriHac 2016 talk: https://www.youtube.com/watch?v=sQxH349HOik
• my SSGEP 2015 lecture notes: https://github.com/kosmikus/SSGEP
• the WGP 2014 talk and paper: https://youtu.be/jzgfM6NFE3Y, https://www.andres-loeh.de/TrueSumsOfProducts/

Possibly also some SO answers such as these (to a large extent these are by @danidiaz):

• https://stackoverflow.com/questions/44193630/how-to-customise-show-read-instances-on-the-lines-of-fromjson-tojson/44195842#44195842
• https://stackoverflow.com/questions/44071503/how-to-map-data-elements-to-data-types/44073088#44073088
• https://stackoverflow.com/questions/43855273/build-more-advanced-generic-parser/43857293#43857293
• https://stackoverflow.com/questions/42692384/how-to-map-over-a-long-tuple-of-instances-of-the-same-class/42705416#42705416
• https://stackoverflow.com/questions/42674354/how-do-i-map-over-parameters/42693764#42693764
• https://stackoverflow.com/questions/42583838/generically-build-parsers-from-custom-data-types/42585429#42585429
• https://stackoverflow.com/questions/41793202/is-it-possible-to-re-implement-enum-deriving-using-ghc-generics/41822243#41822243
• https://stackoverflow.com/questions/41522984/haskell-is-there-a-way-of-mapping-over-an-algebraic-data-type/41524511#41524511
• https://stackoverflow.com/questions/40593441/how-can-i-produce-a-tag-type-for-any-datatype-for-use-with-dsum-without-templat/40621740#40621740
• https://stackoverflow.com/questions/39991581/how-can-i-implement-generalized-zipn-and-unzipn-in-haskell/39997122#39997122
• https://stackoverflow.com/questions/39020787/is-there-a-way-to-apply-maybe-constructor-to-each-field-of-record-with-generics/39027753#39027753
• https://stackoverflow.com/questions/38271220/constructing-haskell-data-types-with-many-fields/38272190#38272190
• https://stackoverflow.com/questions/38248692/whats-a-better-way-of-managing-large-haskell-records/38251692#38251692
• https://stackoverflow.com/questions/36667541/indexing-into-containers-the-mathematical-underpinnings/36672495#36672495
• https://stackoverflow.com/questions/36369414/how-to-generically-extract-all-values-in-haskell-records/36371431#36371431
• https://stackoverflow.com/questions/36254349/retrieving-record-function-in-generic-sop/36255015#36255015
• https://stackoverflow.com/questions/35709737/is-it-possible-to-derive-recursion-principles-generically/35804233#35804233

[kosmikus]

We should also point to at least:

• https://hackage.haskell.org/package/generics-sop-lens
• https://hackage.haskell.org/package/records-sop

and possibly to other more recent reverse dependencies.