docs: add section on ssh-key handling (MR 2251)

Signed-off-by: Robert Eckelmann <longnoserob@postmarketos.org>

docs/index.rst

@@ -20,6 +20,7 @@ For further information, please check out the `postmarketOS-wiki`_.
    usage
    chroot
    debugging
+   ssh-keys
    api/modules
    mirrors
 

docs/ssh-keys.rst

+
+################
+SSH key handling
+################
+
+pmbootstrap can copy SSH keys to the device during the install step.
+
+If the config file option `ssh_keys` is set to `True` (it defaults to `False`),
+then all files matching the glob `~/.ssh/*.pub` will be placed in
+`~/.ssh/authorized_keys` in the user's home directory in newly-built images.
+
+Sometimes, for example if you have a large number of SSH keys, you may wish to
+select a different set of public keys to include in an image. To do this, set
+the `ssh_key_glob` configuration parameter in the pmbootstrap config file to a
+string containing a glob that is to match the file or files you wish to
+include.
+
+For example, a `~/.config/pmbootstrap_v3.cfg` may contain:
+
+.. code-block:: shell
+
+    [pmbootstrap]
+    # ...
+    ssh_keys = True
+    ssh_key_glob = ~/.ssh/postmarketos-dev.pub
+    # ...
+