README.md files for all crate folders

[hawkw]

This would make browsing the repository much nicer. They can probably just be the main doc comment from the crate RustDoc, at least for now.

[hawkw]

• [ ] README foralloc crate
• [x] README for boot crate
• [ ] README for cpu crate
• [x] README for elf crate
• [ ] README for memory crate
• [ ] README for sos_intrusive crate
• [ ] README forutil crate
• [ ] README for vga crate