fbterm: add README (#8241)

plugins/fbterm/README.md

+# fbterm
+
+This plugin automatically starts [fbterm](https://github.com/zhangyuanwei/fbterm)
+if on a real TTY (`/dev/tty*`).
+
+To use it, add fbterm to the plugins array of your zshrc file:
+```
+plugins=(... fbterm)
+```