Establish a process and describe it in the README

As a potential contributor who is interested in contributing to the project, I would like to be able to quickly understand the spirit and state of the project and how can I contribute to it.

I suggest to establish the following things, and for them to be written in or linked from the readme:

## What is the vision/ethos of Haskell School?

Establish what kind of resource Haskell School is, what are the guiding principles and who is the target audience. It should also help answering questions such as:

- Which topics should be covered and which shouldn't (e.g. lambda calculus, template haskell, f-algebras, parsing, arrows, lenses, nix, etc.)
- What style should Haskell School tutorials look like? (reference, prose, concise, have many examples, etc.)
- Is this guide right for me?

## Roadmap

Which content is planned for Haskell School? What is the status of each task? What is the priority of tasks (if there is one)? Can I suggest new topics?

I would suggest creating some sort of a table or a board of the currently planned chapters and give each chapter a status which could be one of these:

- Waiting for a contributor
- Assigned to a contributor
- PR in progress
- PR pending review
- Completed

## How to contribute

Here are a bunch of questions a potential contributor may ask:

- Can I contribute a chapter?
- Can I review PRs?
- What is the process of contributing?
- How do I volunteer?
- What is expected of me (stellar english? Haskell Expert™? finish work in a week? Answer reviewers in minutes?)?
- How will I know if my work is good?
- Will I get help?
- What are the guidelines for this project?
- License of the project and my contribution?
- Is there anything I need to know? (style guide, tech, etc.)