add instructions for setting shell for multi-term

Fixes #5818

layers/+tools/shell/README.org

@@ -10,6 +10,7 @@
   - [[#default-shell][Default shell]]
   - [[#default-shell-position-width-and-height][Default shell position, width, and height]]
   - [[#set-shell-for-term-and-ansi-term][Set shell for term and ansi-term]]
+  - [[#set-shell-for-multi-term][Set shell for multi-term]]
   - [[#width-of-the-shell-popup-buffers][Width of the shell popup buffers]]
   - [[#enable-em-smart-in-eshell][Enable em-smart in Eshell]]
   - [[#protect-your-eshell-prompt][Protect your Eshell prompt]]
@@ -42,6 +43,7 @@ You can find a quick introductions to them [[https://www.masteringemacs.org/arti
 
 To define the default shell you can set the layer variable =shell-default-shell=
 to the following variables:
+
 - =eshell=
 - =shell=
 - =term=
@@ -85,6 +87,15 @@ Default value is =/bin/bash=.
     '((shell :variables shell-default-term-shell "/bin/bash")))
 #+END_SRC
 
+** Set shell for multi-term
+The default shell can be set by setting the variable =multi-term-program=.
+Default value is =/bin/bash=.
+
+#+BEGIN_SRC emacs-lisp
+  (setq-default dotspacemacs-configuration-layers
+    '((shell :variables multi-term-program "/bin/bash")))
+#+END_SRC
+
 ** Width of the shell popup buffers
 By default the popup buffer spans the full width of the current frame, if
 you prefer to spans only the width of the current window then set the