README: document TTY and FDE environment

README.md

@@ -73,6 +73,31 @@ The `ufkbd-wl` build target is for most compatible Wayland compositors.
 Configure your desired keyboard to run on startup. This may mean a desktop file
 in `~/.config/autostart`.
 
+## TTYescape
+
+In the TTY, on-screen keyboards can emit key presses with uinput. A special
+Linux keymap API also allows arbitrary text to be emitted. Backspace might not
+work as expected when typing multi-byte UTF-8 characters.
+
+### Build target
+
+The `ufkbd-evfb` build target is for the TTY.
+
+## Full-disk encryption on postmarketOS
+
+For full-disk encryption, the password is expected on stdout. The keyboard
+needs to process individual key events by itself. It is similar to TTYescape
+where the keyboard is rendered to the framebuffer.
+
+### Build target
+
+The `ufkbd-evfbo` build target is for password entry.
+
+### Switching keyboards
+
+Set the KEYBOARD property in `/etc/conf.d/ttyescape.conf` to your desired
+keyboard.
+
 # Installation
 
 This program has the following build dependencies: