Be a bit more verbose in the README

There is another software called magic out there, so let's call it libmagic from
file(1).

README

@@ -7,11 +7,11 @@ Requirements
 ------------
 gtk2 (>= 2.28)
 girara
-sqlite3 (>= 3.5.9)
+sqlite3 (optional, >= 3.5.9)
 check (for tests)
 intltool
-python-docutils (for man pages)
-magic (optional)
+python-docutils (optional, for man pages)
+libmagic from file(1) (optional, for mime-type detection)
 
 Please note that you need to have a working pkg-config installation and that the
 Makefile is only compatible with GNU make. If you don't have a working