Update Readme

Author: lon-io

README.md

@@ -1,2 +1,4 @@
 ### My Solutions to Algorithm questions on Leetcode
-- [Longest SubString](https://leetcode.com/problems/longest-substring-without-repeating-characters]) => [Solution](https://github.com/lon-io/leetcode_solutions/blob/master/longest_substring.go)
+- [Two Sum](https://leetcode.com/problems/two-sum]) => [two_sum.go](https://github.com/lon-io/leetcode_solutions/blob/master/two_sum.go)
+- [Add Two Numbers](https://leetcode.com/problems/add-two-numbers]) => [add_two_numbers.go](https://github.com/lon-io/leetcode_solutions/blob/master/add_two_numbers.go)
+- [Longest SubString](https://leetcode.com/problems/longest-substring-without-repeating-characters]) => [longest_substring.go](https://github.com/lon-io/leetcode_solutions/blob/master/longest_substring.go)