add a README

There is no README. Write one covering:

• what dew is (a local-first todo.txt TUI + CLI)
• how to build/install it
• where the config lives and what the keys do
• the CLI subcommands (add, do, rm, pin, wait, ls, flat, tui)
• the TUI keybindings, which right now are only discoverable by reading tui.go or pressing ?