Added instructions for best use of KiCad Cheatsheet

(cherry picked from commit 6ebad43429200c1f4b49d30be38425cbf343f1e8)

CHEATSHEET.adoc

@@ -0,0 +1,9 @@
+KiCad Cheatsheet
+================
+
+The KiCad Cheatsheet was created using the Roboto font, usually present
+in every GNU/Linux distribution.
+
+Install this font before trying to print it if you want to obtain the
+best results.
+