Define and communicate goals and non-goals of this project

There are multiple alternative approaches to CosmWasm build systems that come with different priorities and trade-offs.

E.g.

- Supporting private dependencies (https://github.com/CosmWasm/rust-optimizer/pull/82)
- Optimizing for incremental build speed (https://github.com/CosmWasm/rust-optimizer/pull/84)
- Writing an alternative build system (https://github.com/mandrean/cw-optimizoor)

In order to guide future development and be able to confidentially reject PRs that are not in line, we should define goals and non-goals here.