[leetcode.cn] update docs.

Signed-off-by: Eric Wang <skygragon@gmail.com>

Author: skygragon

README.md

@@ -18,6 +18,7 @@
 |[cpp.lint](/docs/cpp.lint.md)|C++ code syntax check|`test`|
 |[cpp.run](/docs/cpp.run.md)|Test C++ code locally|`test`|
 |[github](/docs/github.md)|Commit accpeted code to GitHub|`submit`|
+|[leetcode.cn](/docs/leetcode.cn.md)|Fight questions from leetcode-cn.com||
 |[lintcode](/docs/lintcode.md)|Fight questions from lintcode.com|`list` `show` `test` `submit` `user`|
 |[solution.discuss](/docs/solution.discuss.md)|Fetch top voted solution|`show`|
 

docs/leetcode.cn.md

@@ -0,0 +1,7 @@
+# leetcode.cn
+
+Run everything against "leetcode-cn.com" instead of "leetcode.com".
+
+## Requirement
+
+You need a valid account on leetcode-cn.com since it uses diffrent user database from leetcode.com.