Docs for new contributors

[ChurchOfTheSubgenius]

We need a set of docs explaining the project layout, rust libraries used, design motivation etc. This should be written in such a way that a new Rust dev (who's only read the Rust book, and is familiar with matrix) can get an overall idea of the codebase and start contributing right away.

We can expand this later with design specs for the API etc.

[avdb13]

100% agreed here, finishing a roadmap of issues to get to a stable release is our top priority.
Let's create a list of requirements required to reach 0.0.x, organize them by priority, the crates affected, etc.

I just had a first question round with our new contributor, and we severely lack direction for our project as a lot of questions about the exact scope of the project including what a re-implementation of the Go version should cover.

[avdb13]

What do we exactly need here? I have finished the first few endpoints for the alpha release, I did use comments sparingly for now until a new contributor has a look at the code and can point out where just code doesn't make the logic obvious.

The matrix crate folder structure mirrors the API layout 1:1 i.e. /_matrix/client/v3/createRoom -> src/client/create_room.rs so that the relevan sections can be retrieved more easily.

@marceline-cramer might also be able to help us out here.