Manual.md: document register_shell variable.

Manual.md

@@ -594,6 +594,10 @@ The format is a white space separated pair of strings that represent the name of
 most of the time `pkgname`, and the version of the module, most of the time `version`.
 i.e `dkms_modules="$pkgname $version zfs 4.14"`
 
+- `register_shell` A white space separated list of shells defined by absolute path to be
+registered into the system shells database. It is used by the `register-shell` trigger.
+i.e `register_shell="/bin/tcsh /bin/csh"
+
 <a id="explain_depends"></a>
 #### About the many types of `depends` variable.