[Doc] Add guide as how community can help add more OS support 

From https://community.easyengine.io/t/easyengine-v4-and-whatnot/11397/4

> Why is it EEv4 was released for the already aging OSs and not for the newer Ubuntu 18.01LTS/18.10 or for Debian 9?

```
https://easyengine.io/handbook/contribute/ <new section>
https://easyengine.io/handbook/contribute/improve-platform-support/ <new page>
```