Document how to map GHC AST node to Kythe schema

How to map a small part of GHC AST to Kythe schema. A small tutorial.

- where to look for GHC AST documentation (haddock, github.com/edsko/ghc-dump-tree, maybe some useful article from https://ghc.haskell.org/trac/ghc/wiki)
- where to look for Kythe schema (http://kythe.io/docs/schema/)
- what code needs to be changed (extending Translate.hs data types, mapping from GHC AST to Translate.hs DS in Ghc.hs, adding a new type in Typed.hs, doing the mapping in Kythe.hs). If possible, add a reference to a git branch/push/commit/whatever is kept visible in github with the complete code example of adding a mapping.
- writing unit tests for the mapping (a small example)
- writing kythe verification tests (what they are, where to put them, how to run them, where to get more documentation)