Install personal-flavoured prezto inside dotfiles

I try to make this managable with dotbot

.gitmodules

@@ -4,3 +4,6 @@
 [submodule "external/vim-plug"]
 	path = external/vim-plug
 	url = https://github.com/junegunn/vim-plug.git
+[submodule "external/prezto"]
+	path = external/prezto
+	url = git@github.com:bixel/prezto.git

external/prezto

@@ -0,0 +1 @@
+Subproject commit 2db17c6baa9330b1c1519e04b72a73763d48fa28

install.conf.yaml

@@ -20,3 +20,4 @@
         create: true
         relink: true
         path: external/vim-plug/plug.vim
+    ~/.zprezto: external/zprezto/