Add a comment to install vips library

README.md

@@ -3,6 +3,9 @@
 A Ruby gem for creating static HTML photo galleries from organized image directories. Fotos extracts EXIF data, generates thumbnails, and creates date-organized HTML galleries.
 
 ## Installation
+Install dependant libraries:
+
+    $ apt install libvips42
 
 Install the gem locally: