A small documentation on contribution

Author: Šarūnas Navickas

CONTRIBUTING.md

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+New agents
+==========
+Most natural way of contributing to this project is creating new agents.
+What is agent? It is basically a metrics collector for specific service, 
+eg. `oshino-redis` is an agent which runs `info` command as a Redis client,
+parses received metrics and gives them to `oshino` as events.
+
+To make life easier, there's a `cookiecutter` template (oshino-cookiecutter)[https://github.com/CodersOfTheNight/oshino-cookiecutter]
+which generates all boilerplate code, you just need to modify it.
+
+Already existing custom agents are mostly listed here: (Third party Agents)[https://github.com/CodersOfTheNight/oshino/blob/master/docs/thirdparty.md]
+
+Contributing to the Core
+========================
+It should mostly consist of reporting/solving issues. 
+
+General Guidelines
+===================
+- Code must pass `flake8` (PEP8) coding style test
+- All code (with few exceptions) must be non-blocking - asyncio compatible
+- New parameters should be covered in documentation section