Update README.md for gstreamer

[YapWC]

Add a command for user to check whether gstreamer is installed on their machine.

[Quedale]

Unfortunately this isn't enough and may just lead to more confusion.
Gstreamer is actually a plugin based framework which is distributed under separate components.

Using gst-inspect-1.0 only confirm that gstreamer-tools is installed, but nothing guarantees that all the necessary plugins are available.

I did attempt to document this part, but the major problem is that the components aren't distributed the same way on all platforms. We ultimately need to document it for each individual platforms, which I why I simply refer to using the magic of autogen.sh to search and build it if any is missing.

This work wouldn't be in vain since I ultimately need to this information to make distro-specific deb/rpm packages.

You can find the list of necessary gstreamer plugins on line 1181 if you want to attempt it.