Add install instructions to announce

Author: hadley

ANNOUNCE

@@ -7,6 +7,9 @@ that make plotting a hassle (like drawing legends) as well as
 providing a powerful model of graphics that makes it easy to produce
 complex multi-layered graphics.
 
+To install or update, run:
+install.packages(c("ggplot2", "plyr"))
+
 Find out more at http://had.co.nz/ggplot2, and check out the nearly 500
 examples of ggplot in use.  If you're interested, you can also sign up to 
 the ggplot2 mailing list at http://groups.google.com/group/ggplot2, or track