Missing bits from docs

[oscarbenjamin]

Folllowing comments in gh-32.

The following sections need adding or filling out in docs:

• README
• Frontpage: features and requirements.
• Usage section.
• Dev section in docs
• Overview of core types showing relation between classes.
• Also add an overview of simplecas and explain what its purpose is.
• TreeExpr.children replace dummy docstring.
• ForwardGraph add a good docstring with diagrams.

(Feel free to edit/add to the list)

[asmeurer]

The main thing we need right now is to fill out the docs main page / project README to explain what the project is and what the plans for it are.

[samithkavishke]

I would like to help on this, can you share some reference materials if possible or guidance to understand the repository.