Document hidden HOOK_BANNER

Signed-off-by: Joachim Wiberg <troglobit@gmail.com>

doc/bootstrap.md

@@ -1,6 +1,8 @@
 Bootstrap
 =========
 
+0. Get bearings, disable kernel logs, initialize TTY and print banner,
+   unless `HOOK_BANNER` is defined, then call that instead
 1. Prepare emergency shell, if enabled
 2. Set up initial signal handlers
 3. Set up default $PATH early

doc/plugins.md

@@ -80,6 +80,11 @@ Hooks
 
 ### Bootstrap Hooks
 
+* `HOOK_BANNER`: The very first point at which a plugin can run.
+  Intended to be used as a banner replacement.  Essentially this runs
+  just before entering runlevel S.  Assume nothing is available, so
+  be prepared to use absolute paths, etc.
+
 * `HOOK_ROOTFS_UP`: When `finit.conf` has been read and `/` has is
   mounted — very early