update readme

Author: hackw

README.md

@@ -1,5 +1,7 @@
 # Flutter ConstraintLayout
 
+[简体中文](https://github.com/hackware1993/flutter-constraintlayout/blob/master/README_CN.md)
+
 A super powerful Stack, build flexible layouts with constraints. Similar to ConstraintLayout for
 Android and AutoLayout for iOS.
 
@@ -115,7 +117,8 @@ class NestedHell extends StatelessWidget {
        of the undetermined side will be calculated using the determined side based on the aspect
        ratio. The undetermined side must be matchConstraint, and the determined side can be
        matchParent, fixed size(>=0), matchConstraint)
-14. relative id(if an id is defined for a child element, it cannot be referenced using a relative id)
+14. relative id(if an id is defined for a child element, it cannot be referenced using a relative
+    id)
     1. rId(3) represents the 3th child element, and so on
     2. sId(-1) represents the previous sibling element, and so on
     3. sId(1) represents the next sibling element, and so on