tracing: Add documentation for trace event triggers

Provide a basic overview of trace event triggers and document the
available trigger commands, along with a few simple examples.

Link: http://lkml.kernel.org/r/2595dd9196d7b553049611f2a3f849ca75d650a2.1382622043.git.tom.zanussi@linux.intel.com



Signed-off-by: Tom Zanussi <tom.zanussi@linux.intel.com>
Signed-off-by: Steven Rostedt <rostedt@goodmis.org>