Add contribution guide.

CONTRIBUTING.md

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+## Contributing
+
+#### 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
+
+- 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
+
+- 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:
+    - **Source code** with comments and readable namings
+    - **Math** being explained in README.md along with the code
+    - **Jupyter demo notebook** with example of how this new algorithm may be applied