Enum sum generics example to test/docs

[justinwoo]

https://github.com/justinwoo/enum-sum-generics-example-simple-json/blob/master/src/Main.purs

Should be copied into test/Generic.purs or a new test/EnumGeneric.purs file and then added to docs

[justinwoo]

Now there's an example: https://github.com/justinwoo/purescript-simple-json/blob/master/test/EnumSumGeneric.purs

[justinwoo]

I should probably write up some stuff about this.

[justinwoo]

Fixed by 5f31f39 add notes on enum sums