Add brief introduction to functional programming in the elixir-introduction

[riverrun]

This proposal is to add a brief outline of certain principles / benefits of function programming near the beginning of the elixir-introduction file.

This might provide users with pointers to keep in mind while they read the rest of the book - especially if they are used to languages that belong to other paradigms.

Further details

We should mention the following:

• functions as first-class citizens and higher-order functions
• immutability
• disciplined state
  • easier to run code in parallel

The already existing sections on functions as first-class citizens and immutability could be simplified and moved to this introduction.

[riverrun]

On reflection, I think it might be more straightforward to explain some of these concepts as we encounter them (for example, we can mention higher-order functions when looking at anonymous functions).