Update Solrini documentation for Washington Post

[lintool]

@x389liu if you've successfully ingested Washington Post in Solr, can you add some documentation to that effect at the bottom of: https://github.com/castorini/anserini/blob/master/docs/solrini.md

Something like, we've verified that these instructions can be straightforwardly adapted to work with Washington Post based on instructions at https://github.com/castorini/anserini/blob/master/docs/regressions-core18.md

[x389liu]

PR #817 created.

[lintool]

Closing, as #817 has been merged.