Add contribution guide.

CONTRIBUTING.md

@@ -1,16 +1,16 @@
 ## Contributing
 
-#### General Rules
+### General Rules
 
 - As much as possible, try to follow the existing format of markdown and code.
 - Don't forget to run `pylint ./homemade` before submitting pull requests.
 
-#### Contributing New Translation
+### Contributing New Translation
 
 - Create new `README.xx-XX.md` file with translation alongside with main `README.md` file where `xx-XX` is [locale and country/region codes](http://www.lingoes.net/en/translator/langcode.htm). For example `en-US`, `zh-CN`, `zh-TW`, `ko-KR` etc.
 - You may also translate all other sub-folders by creating related `README.xx-XX.md` files in each of them.
 
-#### Contributing New Algorithms
+### Contributing New Algorithms
 
 - Make your pull requests to be **specific** and **focused**. Instead of contributing "several algorithms" all at once contribute them all one by one separately (i.e. one pull request for "Logistic Regression", another one for "K-Means" and so on).
 - Every new algorithm must have: