feat: update readme to give ast content

Author: 崔竞宁

README.md

@@ -1,5 +1,5 @@
 # go-querystring-parser
-A golang querystring parser
+A golang Elasticsearch Querystring parser
 
 ## Usage
 
@@ -21,6 +21,25 @@ if err != nil {
 // do something with ast
 ```
 
+The AST parsed from above querystring:
+
+```
+// print via github.com/davecgh/go-spew.Dump
+(*querystring.AndCondition)(0xc00000c200)({
+    Left: (*querystring.MatchCondition)(0xc00000c1e0)({
+        Field: (string) (len=7) "message",
+        Value: (string) (len=10) "test value"
+    }),
+    Right: (*querystring.TimeRangeCondition)(0xc000076750)({
+        Field: (string) (len=8) "datetime",
+        Start: (*string)(0xc0000545b0)((len=19) "2020-01-01T00:00:00"),
+        End: (*string)(0xc0000545c0)((len=19) "2020-12-31T00:00:00"),
+        IncludeStart: (bool) true,
+        IncludeEnd: (bool) true
+    })
+})
+```
+
 ## For Developers
 
 After edit querystring.y, gen code via run: