Extend README Documentation

Currently the [README.md](https://github.com/dwyl/learn-elixir/blob/master/README.md) gives a good introduction by covering the basics. I feel as though this should be extended further to include more things integral to a functional programming language:

- immutability
- recursive functions
- partial application
- heads and tails

Does anyone have any thoughts on things to include?

Initial list of topics to add or extend within the documentation:

- [x] Generating an elixir project
- [ ] Object Oriented vs Functional
- [ ] Native Modules
- [x] Module Documentation
- [x] Testing
- [ ] Pattern Matching - extend #57
- [ ] Elixir -> Erlang -> BEAM
- [x] Pipe Operator
- [x] Maps
- [ ] Keyword Lists
- [ ] Structs