Update build documentation to include extra plugin installation instructions.

[scribblemaniac]

Issue Summary

The current build documentation does not have any information about how to enable ffmpeg support on all platforms or SSL support on Windows.

Expected Results

There should be documentation on the wiki and in docs/ detailing where to get prebuilt binaries for ffmpeg and the dlls for Windows SSL support, as well as where to place those files in order for those features to work correctly. Currently, information about all of this can be worked out by reading our automated build scripts to see what they do before deployment.

See brief discussion in #1216