Introduce Rust guide

[natelust]

This is a first draft of the proposed rust developer guide. All comments and improvement welcome.

[TallJimbo]

Two big-picture comments (some echoed in various line-comments):

• I think you need to mention rubinoxide up front concretely and organize the discussion around that; only talking about organizing code in a purely abstract sense makes the actual guidelines a lot harder to follow.
• There are a lot of start-of-section sentences here that just sound like software platitudes. I understand wanting to have some sort of introductory text for each section, but I think the audience (who is likely reading it as fast as they can so they can get back to writing code) will appreciate it if you just dive right in.