Draft SolrFacetingOverview page added to Solr wiki

[J.J. Larrea]

Hello, all.

I added a draft page http://wiki.apache.org/solr/SolrFacetingOverview 
to the wiki, for your perusal, comments, and direct editing. 
Hopefully it can help answer some of the questions on faceting which 
frequently come up on solr-users.

It's not currently linked into the FrontPage; I'll so once the dust 
settles on any glaring errors or omissions I may have made, opinions 
on whether it should remain as a standalone page (rather than 
incorporated directly into SimpleFacetParameters), better naming, and 
so forth.

- J.J. Larrea