user_events: Add documentation file

Add a documentation file about user_events with example code, etc.
explaining how it may be used.

Link: https://lkml.kernel.org/r/20220118204326.2169-13-beaub@linux.microsoft.com



Acked-by: Masami Hiramatsu <mhiramat@kernel.org>
Signed-off-by: Beau Belgrave <beaub@linux.microsoft.com>
Signed-off-by: Steven Rostedt (Google) <rostedt@goodmis.org>