Mention system codecs on Tor Browser's Linux requirements

Tor Browser doesn't ship code for dealing with patented codecs (h.264, and so on) for legal reasons. We even disabled h.265 for that.

However, some users will find this incompatibility surprising.

Probably we should mention that Firefox can use some libraries from your system when available (libavcodec I believe?).

@boklm you probably know more about this, since you added suggestions for our deb packages.