message: Added proper references for Apache Lucene & Solr
  Also, added links to websites for Lucene, Solr & ElasticSearch
diff: **Tantivy** is a **full text search engine library** written in rust.
  -It is closer to Lucene than to Elastic Search and Solr in the sense it is not
  +It is closer to [Apache Lucene](https://lucene.apache.org/) than to [Elastic Search](https://www.elastic.co/products/elasticsearch) and [Apache Solr](https://lucene.apache.org/solr/) in the sense it is not
  an off-the-shelf search engine server, but rather a crate that can be used
  to build such a search engine.
  