documentation: explain file configuration format

README.md

@@ -31,6 +31,32 @@ The configuration file describes the size of the memory filesystem,
 the list of files and the list of directories that should be added
 to the filesystem as symbolic links from the persistent directory.
 
+There are currently three keys:
+- **size**: which is a parameter to mount_mfs -s to give the ramdisk size
+- **files**: which is a list of files relative to $HOME
+- **directores**: which is a list of directories relative to $HOME
+
+Minimalistic example of `/home/persist/my-user/impermanence.yml`:
+
+```
+size: 200m
+files:
+  - .bashrc
+  - .gitconfig
+  - .profile
+  - .tmux.conf
+  - .xsession
+directories:
+  - .config
+  - .local/share
+  - .mozilla
+  - .ssh
+  - Data
+  - Documents
+  - Downloads
+  - dev
+```
+
 # home-impermanence rc service
 
 ## restart