feat(provider-sql): add README.md

Author: andanyoung

dict-provider-sql/src/main/java/com/admin4j/dict/provider/sql/impl/PropertiesSqlDictService.java

@@ -19,9 +19,9 @@
 public class PropertiesSqlDictService implements SqlDictService {
 
 
-    private final SqlDictProperties sqlDictProperties;
+    protected final SqlDictProperties sqlDictProperties;
 
-    private final SqlDictManager sqlDictManager;
+    protected final SqlDictManager sqlDictManager;
 
     /**
      * 通过 字典code 获取字典显示值