Document general design of project

[bahildebrand]

Per a request in #274 we should document the general design of the project. One key issue that needs to be touched on specifically is mentioned below:

A minimum documentation describing the idea behind the current design. Other runtimes are simpler whereas 
Rust's wants to provide more than just an execution entry point (namely, lambda-http).

[calavera]

👋 I'm going to close this ticket, unless someone still really wants that document. I think we've simplified the experience enough for this to not be necessary anymore. For example, creating extensions is almost trivial, which not every language can say. Feel free to comment if you think we should still keep this open.