Update Readme: Details relating to running on Linux

[LRTNZ]

When setting up Caspar the other day to run on a new Linux server, I ran into a bit of an issue. I could not get it to work properly. It was not until poking in the source code, that I realised the issue was that I had not installed FFmpeg onto the new machine.
So, tldr; Can we please have the Readme updated, to say that on Linux having the apt-get package of FFmpeg installed is a requirement?

[dotarmin]

@LRTNZ, it would be appreciated if you can do a PR.

Thanks in advance

[LRTNZ]

@dotarmin Sweet, will do that later this morning. On that note, I was wondering if I should pull the Linux usage section out into its own part, to add in the chown fix/workaround relating to the ownership of the media folder - #28 ?

[LRTNZ]

@dotarmin I put in PRs for this a few days back, as well as a minor fix to stop the scanner binding to IPv6 ports.