Add README

Author: Thuy Trinh

src/main/java/leetcode/flattenbtree/README.md

@@ -0,0 +1,26 @@
+```
+  1
+ / \
+2   3
+```
+
+* Temporarily hold `3`.
+* Change `1`'s right tree into `2`. Clear `1`'s left tree.
+* Change `2`'s right tree into `3`. Clear `2`'s left tree.
+
+```
+    1
+   / \
+  2   3
+ /   /
+4   6
+```
+
+* Flatten the subtree `2`.
+* Flatten the subtree `3`.
+* Temporarily hold root of the subtree `3`.
+* Change `1`'s right tree into root of the subtree `2`. Clear `1`'s left tree.
+* Get last node of the subtree `2`. Change the right node of this last node into root of the subtree `3`.
+* Return last node of the flattened left subtree `2` if any.
+* Return last node of the flattened right subtree `3` if any.
+* Otherwise, return `1`.