mention library packages needed

[jidanni]

$ make
gcc sound_meter.c -o sound_meter -lm -lasound
sound_meter.c:25:9: fatal error: alsa/asoundlib.h: No such file or directory
   25 | #include<alsa/asoundlib.h>
      |         ^~~~~~~~~~~~~~~~~~
compilation terminated.

[jidanni]

Need to apt-get install libasound2-dev