修复一些错误并更新README

ExpressionElements/MemberElements/Identifier.cs

@@ -15,7 +15,9 @@ using System;
 
 namespace Flee.ExpressionElements.MemberElements
 {
-    [Obsolete("Represents an identifier")]
+    /// <summary>
+    /// 表示标识符的表达式元素
+    /// </summary>
     internal class IdentifierElement : MemberElement
     {
         private FieldInfo _myField;