improved usage info

roles/extra_packages/README.md

 This role adds all the packages we think are useful but aren't clearly a dependency of some function.
+
+Before calling, you may want to  define the following lists:
+
+extra_packages  #list of yum packages. Or see code on how to it loads an  alternative file  if not defined
+exclude 	#list of packages to exclude
+useEpel 	# if defined, it will enable epel repo
+
+
+Usage:
+  roles:
+  - { role: extra_packages, useEpel: True, tags: [ other, extra_packages ] }