Add 'fb' subcommand to render framebuffer

[joukos]

• Minimalistic feature to draw framebuffer contents on the e-ink
• Basically copy-pasted from the VNC feature
• Quick README update to hopefully make installation more obvious

[joukos]

Merging this as it should marginally improve the very stale docs, and adds an important (yet barebones) framebuffer feature so people can try it out.