added a documentation for match phrase prefix query (#215)

docs/Query/FullText/MatchPhrasePrefix.md

-Documentation in progress...
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+# Match Phrase Prefix Query
+
+> More info about match phrase prefix query is in the [official elasticsearch docs][1]
+
+The `match_phrase_prefix` is the same as `match_phrase`, except that it allows for prefix matches on the last term in the text. For example
+
+## Simple example
+
+```JSON
+{
+    "query": {
+        "match_phrase_prefix" : {
+            "message" : {
+                "query" : "quick brown f",
+                "max_expansions" : 10
+            }
+        }
+    }
+}
+```
+
+In DSL:
+
+```php
+$query = new MatchPhrasePrefixQuery('message', 'quick brown f');
+$query->addParameter('max_expansions', 10);
+
+$search = new Search();
+$search->addQuery($query);
+
+$queryArray = $search->toArray();
+```
+
+[1]: https://www.elastic.co/guide/en/elasticsearch/reference/current/query-dsl-match-query-phrase-prefix.html