Upload a new file for crawling and indexing

1. ​POST https://opensolr.com/solr_manager/api/index_crawler_file
2. Parameters:
   1. ​email - your opensolr registration email address
   2. api_key - your opensolr api_key
   3. core_name - the name of the core you wish to upload the document for
   4. url - optionally, you can add an URL to crawl and index, alongside the uploaded document. They will both be indexed as separate documents in your opensolr index.
   5. userfile - your local document file to POST and upload to the server (pdf, doc, html, xls, mp3, mp4, etc.)
3. Example here: https://opensolr.com/solr_manager/index_crawler_file