go.blog: add tags to race detector article

R=golang-dev, dsymonds
CC=golang-dev
https://golang.org/cl/10585043
diff --git a/content/race-detector.article b/content/race-detector.article
index 28c0d08..6dfbb5a 100644
--- a/content/race-detector.article
+++ b/content/race-detector.article
@@ -1,5 +1,6 @@
 Introducing the Go Race Detector
 26 Jun 2013
+Tags: concurrency, technical
 
 Dmitry Vyukov