update README

here is an update to the README file. I'd like to stop using the term "filer" or "vfiler" or "virtual_filer" as those terms are specific to 7-mode, whereas this cookbook is for clustered mode.

Note that while this updates the README, there is an implied code change to convert the attributes:

['netapp']['virtual_filer'] string

to 

['netapp']['vserver'] string