documenting ParseContext

[Allison, Timothy B.]

All,
  On SOLR-9178[1], Alexandre Rafalovitch asked about documentation/uses of 
ParseContext.  I started a page on our wiki: 
https://wiki.apache.org/tika/TikaParseContext  Please edit as you see fit.
  Thank you.

                Best,

                       Tim

[1] https://issues.apache.org/jira/browse/SOLR-9178