* Spice up the project README, fill out more docs
This is an attempt to spruce up wasmtime's README.md file as well as
fill out more of the missing documentation in the `docs/` folder.
There's still a long way to go but I hoped here to mostly move around
existing information and add new information. As always happy to have
feedback!
* Tweak CLI wording
* Remove no-longer relevant clause
* Update sandboxing docs
* Handle comments
