Add link to troubleshooting guide in UP4

Change-Id: I18c6ae3f63a3e38e0dcd66df1692c08bc2045318
diff --git a/troubleshooting.rst b/troubleshooting.rst
index fdc0764..222c934 100644
--- a/troubleshooting.rst
+++ b/troubleshooting.rst
@@ -1,3 +1,5 @@
+.. _troubleshooting_guide:
+
 Troubleshooting Guide
 =====================