Add bool filter doc.

docs/Filter/Bool.md

+# Bool Filter
+
+> More info about filter query is in the [official elasticsearch docs][1]
+
+A filter that matches documents matching boolean combinations of other queries. Similar in concept to
+[Boolean query][2], except that the clauses are other filters.
+
+To create a bool filter unlike other queries you don't have to create `BoolFilter` object. Just add queries to the
+search object and it will form bool filter automatically.
+
+Lets take an example to write a bool query with Elasticsearch DSL.
+
+```JSON
+{
+    "filtered" : {
+        "query" : {
+            "queryString" : {
+                "default_field" : "message",
+                "query" : "elasticsearch"
+            }
+        },
+        "filter" : {
+            "bool" : {
+                "must" : {
+                    "term" : { "tag" : "wow" }
+                },
+                "must_not" : {
+                    "range" : {
+                        "age" : { "gte" : 10, "lt" : 20 }
+                    }
+                },
+                "should" : [
+                    {
+                        "term" : { "tag" : "sometag" }
+                    },
+                    {
+                        "term" : { "tag" : "sometagtag" }
+                    }
+                ]
+            }
+        }
+    }
+}
+```
+
+And now the query via DSL:
+
+```php
+$queryStringQuery = new QueryStringQuery('elasticsearch', ['default_field' => 'message']);
+
+$termFilter1 = new TermFilter('tag', 'wow');
+$rangeFilter = new RangeFilter('age', ['gte' => 10, 'lt' => 20]);
+$termFilter2 = new TermFilter('tag', 'sometag');
+$termFilter3 = new TermFilter('tag', 'sometagtag');
+
+$search = new Search();
+$search->addQuery($queryStringQuery);
+$search->addFilter($termFilter1);
+$search->addFilter($rangeFilter, BoolFilter::MUST_NOT);
+$search->addFilter($termFilter2, BoolFilter::SHOULD);
+$search->addFilter($termFilter3, BoolFilter::SHOULD);
+
+$queryArray = $search->toArray();
+```
+
+There is also an exception due adding filters to the search. If you add only one filter without type
+it will form simple query. e.g. lets try to create match all filter.
+
+```php
+$matchAllFilter = new MatchAllFilter();
+
+$search = new Search();
+$search->addFilter($matchAllFilter);
+
+$queryArray = $search->toArray();
+```
+
+You will get this query:
+```JSON
+{
+    "query": {
+        "filtered": {
+            "filter": {
+                "match_all": {}
+            }
+        }
+    }
+}
+```
+
+> More info about `Search` look in the [How to search](../HowTo/HowToSearch.md) chapter.
+
+
+
+[1]: https://www.elastic.co/guide/en/elasticsearch/reference/current/query-dsl-bool-filter.html
+[2]: ../Query/Bool.md
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