Update README.md

Author: bofeizhu

README.md

@@ -14,6 +14,7 @@
 ## List of Companies
 You can visit the pages below to search problems by company tags. Then come back and use ⌘ + F to search by problem name or problem #. **(LeetCode subscription is required to visit these pages)**
 - [Google](https://leetcode.com/company/google/)
+- [Facebook](https://leetcode.com/company/facebook/)
 
 ## List of Problems & Solutions
  \# | Problem  | Solution | Approach