options: Describe --key SELECTOR in a single file.

Other man pages can include this file to document the --key option and
selector.

Author: rwmjones

options/key-option.pod

@@ -0,0 +1,16 @@
+=item B<--key> SELECTOR
+
+Specify a key for LUKS, to automatically open a LUKS device when using
+the inspection.
+
+=over 4
+
+=item B<--key> C<DEVICE>:key:KEY_STRING
+
+Use the specified C<KEY_STRING> as passphrase.
+
+=item B<--key> C<DEVICE>:file:FILENAME
+
+Read the passphrase from F<FILENAME>.
+
+=back