Add mention of trace2html

ninjatracing results are most easily viewed by running them through
trace2html and then loading the generated html file. This change
merely updates README.md to make this discoverable.

README.md

@@ -19,4 +19,8 @@ Idea from Nick Carter, initial implementation from Richard Smith.
 
     (When using --showall, ideally rm $BUILDDIR/.ninja_log and do a clean build.
     If you don't have time for a clean build, at least run
-    `ninja -C $BUILDDIR -t recompact` first to remove no-longer-built targets.)
+    `ninja -C $BUILDDIR -t recompact` first to remove no-longer-built targets.)
+
+    The results can be converted from .json format to .html format, for easier
+    loading into about:tracing, using trace2html from:
+    https://github.com/catapult-project/catapult/blob/master/tracing/bin/trace2html