Add openSUSE prerequisite instructions

These instructions were tested on openSUSE Tumbleweed and openSUSE Leap
15.1. There are some modules left that are not documented yet, but this
already improves the sitution for common openSUSE users.

modules/email/mu4e/README.org

@@ -4,16 +4,18 @@
 #+STARTUP: inlineimages
 
 * Table of Contents :TOC:
-- [[Description][Description]]
-  - [[Module Flags][Module Flags]]
-  - [[Plugins][Plugins]]
-- [[Prerequisites][Prerequisites]]
-  - [[MacOS][MacOS]]
-  - [[Arch Linux][Arch Linux]]
-- [[Features][Features]]
-- [[Configuration][Configuration]]
-  - [[offlineimap][offlineimap]]
-  - [[mbsync][mbsync]]
+- [[#description][Description]]
+  - [[#module-flags][Module Flags]]
+  - [[#plugins][Plugins]]
+- [[#prerequisites][Prerequisites]]
+  - [[#macos][MacOS]]
+  - [[#arch-linux][Arch Linux]]
+  - [[#nixos][NixOS]]
+  - [[#opensuse][openSUSE]]
+- [[#features][Features]]
+- [[#configuration][Configuration]]
+  - [[#offlineimap][offlineimap]]
+  - [[#mbsync][mbsync]]
 
 * Description
 This module makes Emacs an email client, using ~mu4e~.
@@ -66,6 +68,15 @@ environment.systemPackages = with pkgs; [
 
 [[https://github.com/Emiller88/dotfiles/blob/master/modules/shell/mail.nix][An example of setting up mbsync with home-manager]]
 
+** openSUSE
+
+Remove ~#~ in ~#sync_program=offlineimap~ to choose ~offlineimap~ instead of ~mbsync~.
+#+BEGIN_SRC sh :dir /sudo::
+sync_program=isync # mbsync
+#sync_program=offlineimap
+sudo zypper install maildir-utils $sync_programm
+#+END_SRC
+
 * TODO Features
 
 * Configuration