Add rundependencies to README.md

[tpruzina]

Could you add all runtime dependencies to README?
Had to go to source code for stuff like:

static const char* COMMAND_PAMAN = "paman";
static const char* COMMAND_PAVUCONTROL = "pavucontrol";
static const char* COMMAND_PAVUMETER = "pavumeter";
static const char* COMMAND_PAVUMETER_REC = "pavumeter --record";
static const char* COMMAND_PAPREFS = "paprefs";

[christophgysin]

pasystray does not depend on them. But I'm happy to merge a PR documenting them.