Add testing / contributing / roadmap information to readme

[kamathba]

The top-level README for this module is very useful for getting started but could have a few more links for people coming into the project from any direction (rust embedded, zephyr background, no experience, etc). Some of this could already be planned or in progress but I didn't find it (in which case, sorry for repeat)

Proposed changes

• Some sort of doc link to latest gh-pages zephyr project documentation
• Add a roadmap header, could link to original RFC, as well as follow on RFCs plus maybe any other links to inform peoples understanding of where the project is headed.
• Add a testing header, could start with something as simple as tl;dr ./zephyr/scripts/twister -T modules/lang/rust/tests/ plus maybe a link that discusses rustup / toolchain prereqs
• Add a contributing header, that lists all the places where discussion for development work happens for this project. This is actually the thing I was most interested in and the other ideas I thought just might be related / useful for people looking at this for the first time. I have rust and zephyr rtos experience and outside of looking at issues, was wondering what other planning infrastructure exists (message boards, chat / irc, etc)