updated the readme with submodule update instructions

README.md

@@ -8,6 +8,21 @@ The folder 'plugins' is linked to the Pelican project '[pelican-plugins](https:/
 
 `git clone https://git.vvvvvvaria.org/varia/varia.manual.git --recurse-submodules`
 
+## Update submodules
+
+We are currently using two submodules:
+
+* plugins ([https://github.com/getpelican/pelican-plugins](https://github.com/getpelican/pelican-plugins))
+* plugins-custom/events-ics ([https://git.vvvvvvaria.org/rra/events-ics](https://git.vvvvvvaria.org/rra/events-ics)) 
+
+To update these submodules within your (already) cloned repository, first init the submodule, and then update it:
+
+    cd /path/to/submodule
+
+    git submodule init
+
+    git submodule update
+
 ## Posting Content
 We're using a platform called pelican to generate static websites. The main way to add content to the website is by making 'posts' in the folder `content`