[R] update to posting guide: use 'sessionInfo()' instead of 'version'

[Tony Plate]

Some changes have been made to the posting guide, based on suggestions 
from various R-help contributors over the past year.

The most significant change is the recommendation to use 'sessionInfo()' 
  rather than 'version' when asking questions about unexpected behavior 
or bugs.  This change was made because 'sessionInfo()' reports the 
version and a list of packages currently attached.  As more and more 
packages become available, it becomes more likely that unexpected 
behavior is due to conflicts between packages, so this is relevant 
information.

[Note that sessionInfo() currently does not report all the information 
that 'version' does (it omits at least "Status" and "svn rev").  R-core 
members are aware of this -- whether or not they change this is up to them.]

-- Tony Plate