Mention that fortune has to be installed :s

README.md

 ### To install
 `./install.sh`
 
+Also make sure `/usr/games/fortune` exists
+(for example, on Ubuntu nowadays you run `sudo apt install fortune-mod`
+in order to install it).
+
 ### To run
 `/PATH/TO/makefortune.sh` would generate `/PATH/TO/fortune.rss`.