docs: rename SOLUTION_TREE to solution/README.md

See https://github.com/doocs/leetcode/tree/master/solution/README.md

Author: yanglbme

README.md

@@ -18,7 +18,7 @@
 Complete [solutions](/solution) to [LeetCode problems](https://leetcode-cn.com/problemset/all/), updated daily.
 
 ## Solution Tree
-See [SOLUTION_TREE](/SOLUTION_TREE.md).
+See [SOLUTION_TREE](/solution/README.md).
 
 ## Contributions
 I'm looking for long-term contributors/partners to this repo! Send me [PRs](https://github.com/doocs/leetcode/pulls) if you're interested! See the following:

SOLUTION_TREE.md renamed to solution/README.md

@@ -1,4 +1,5 @@
-## Solution Tree
+# Solution Tree
+
 ```
 .
 ├── 0001.Two Sum