added documentation for span first query (#221)

docs/Query/Span/SpanFirst.md

-Documentation in progress...
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+# Span First Query
+
+> More info about Span First query is in the [official elasticsearch docs][1]
+
+Matches spans near the beginning of a field. The span first query maps to Lucene SpanFirstQuery. Here is an example:
+
+```JSON
+{
+    "query": {
+        "span_first" : {
+            "match" : {
+                "span_term" : { "user" : "kimchy" }
+            },
+            "end" : 3
+        }
+    }
+}
+```
+
+And now the query via DSL:
+
+```php
+$search = new Search();
+$query = new SpanFirstQuery(new SpanTermQuery('user', 'kimchy'), 3);
+$search->addQuery($query);
+
+$search->toArray();
+```
+
+
+[1]: https://www.elastic.co/guide/en/elasticsearch/reference/current/query-dsl-span-first-query.html