edited Contributing.md to add 'doc_xxx' (keon#392)

Included language inviting contributors to improve documentation.

Author: feygon

CONTRIBUTING.md

@@ -13,6 +13,7 @@ agree to abide by the [Code of Conduct](CODE_OF_CONDUCT.md).
   * add_XXX if you will add new algorithms or data structures.  
   * fix_XXX if you will fixe a bug on a certain algorithm or data structure.  
   * test_XXX if you wrote a test/s.  
+  * doc_XXX if you added to or edited documentation.
 
 You may contribute by:
 - implementing new algorithms in the repo. Be sure to keep it under
@@ -23,6 +24,7 @@ it doesn't fall under any section. Make sure that your implementation works.
 - finding and fixing bugs.
 - adding examples to explain the algorithms better.
 - adding test cases.
+- improving documentation.
 
 ## Pull Requests
 Push to your fork and [submit a pull request][pr].