docs: format external links

readme.md

@@ -13,10 +13,10 @@ from the user's home directory under "Pictures/wallpaper/" (e.g. "/home/adam/Pic
   - [Change Wallpaper Directory](#change-wallpaper-directory)
   - [Change Wallpaper Duration](#change-wallpaper-duration)
 - [Reference](#reference)
-  - [Available picture-options](#available-picture-options)
+  - [Wallpaper Options](#wallpaper-options)
 - [External Links](#external-links)
 
-**Note:** Verified to work on Arch Linux with GNOME v3.14 to v42.
+**Note:** Verified to work on Arch Linux with GNOME v3.14 to v43.
 
 ## Installation
 
@@ -140,9 +140,11 @@ This alteration will set the duration of how long the wallpaper is displayed for
 
 ## Reference
 
-Not applicable at this time. Here for future use.
+### Wallpaper Options
 
-### Available picture-options
+Desktop wallpaper can be displayed in the following ways (org.gnome.desktop.background.picture-options).
+
+**Note:** Not applicable at this time. Here for future use.
 
 - none
 - wallpaper
@@ -152,9 +154,18 @@ Not applicable at this time. Here for future use.
 - zoom
 - spanned
 
+Acceptable display setting values are sourced from:
+
+```console
+$ gsettings describe org.gnome.desktop.background picture-options
+```
+```console
+Determines how the image set by wallpaper_filename is rendered. Possible values are “none”, “wallpaper”, “centered”, “scaled”, “stretched”, “zoom”, “spanned”.
+```
+
 ## External Links
 
- * https://wiki.archlinux.org/index.php/Systemd/User
+ * [Gsettings with cron - Stack Overflow](https://stackoverflow.com/questions/10374520/gsettings-with-cron)
- * https://wiki.archlinux.org/index.php/Systemd/Timers
+ * [Rotate GNOME 3’s wallpaper with systemd user units and timers - Major Hayden](https://major.io/2015/02/11/rotate-gnome-3s-wallpaper-systemd-user-units-timers/)
- * https://major.io/2015/02/11/rotate-gnome-3s-wallpaper-systemd-user-units-timers/
+ * [systemd/Timers - ArchWiki](https://wiki.archlinux.org/index.php/Systemd/Timers)
- * https://stackoverflow.com/questions/10374520/gsettings-with-cron
+ * [systemd/User - ArchWiki](https://wiki.archlinux.org/index.php/Systemd/User)