hydra-cli not supported on Windows.

[jamiehewitt15]

The hydra-cli currently does not work on Windows, this is the cause of of the following two issues: https://github.com/Joystream/hydra/issues/219 and https://github.com/Joystream/hydra/issues/216

We should look into the cause of this so that we can support Windows in future. In the meantime I will add a note to the readme that warns Windows users that they may experience issues.

[jamiehewitt15]

I have added "Unix based operating system" to the list of prerequisites in this pull request to inform Windows users that they may experience issues: https://github.com/Joystream/hydra/pull/222