Document difference between macro/generator libraries

Differences between:

Scala macro/generator libraries:
- scalamacros/paradise
- scalameta/paradise
- scalacenter/macros

Scala meta-programming constructs:
- def macros
- annotation macros
- annoation generators

Other code generators and metaprogramming tools
- Swift Generators
- Lisp Macros
- Template Haskell
- etc. etc.
