Flesh out README to explain how to create a CDK

[dansteren]

The readme currently focuses on building an AbstractCanisterTree. However, in addition to creating an ACT, CDK authors need to also generate a Rust project including Cargo.toml and Cargo.lock files. There are lots of other nuances to this process. After this ticket is complete a potential CDK author should understand how this process works in both Azle and Kybra and should understand the general recommended steps.