Update README

- More precise compile instructions.
- Pre-required libs.
- Example of handling Alsa volume on host system.
- Explanation of how to write structure file for the alsa plugin.

Change-Id: I8c16b84e2c7407a783fd04fcfe62fe062ce02cc4
Signed-off-by: Mattijs Korpershoek <mattijsx.korpershoek@intel.com>