documentation: Add links to TRACE_EVENT documentation

Existing tracepoint documentation doesn't mention the popular
TRACE_EVENT
macro. Since an excellent series of articles on proper usage already
exists, respective links are added to the existing documentation.

Signed-off-by: Stefan Raspl <raspl@linux.vnet.ibm.com>

---
 Documentation/trace/tracepoints.txt |    5 +++++
 1 file changed, 5 insertions(+)

--- a/Documentation/trace/tracepoints.txt
+++ b/Documentation/trace/tracepoints.txt
@@ -99,3 +99,8 @@ core kernel image or in modules.
 If the tracepoint has to be used in kernel modules, an
 EXPORT_TRACEPOINT_SYMBOL_GPL() or EXPORT_TRACEPOINT_SYMBOL() can be
 used to export the defined tracepoints.
+
+Note: The convenience macro TRACE_EVENT provides an alternative way to
+      define tracepoints. Check http://lwn.net/Articles/379903,
+      http://lwn.net/Articles/381064 and http://lwn.net/Articles/383362
+      for a series of articles with more details.