Document ignoreContextHeader configuration (#296)

PR-URL: #296

config.js

@@ -81,7 +81,13 @@ module.exports = {
     //            the network.
     onUncaughtException: 'ignore',
 
-    // Allows to ignore the requests X-Cloud-Trace-Context -header if set
+    // EXPERIMENTAL:
+    // Allows to ignore the requests X-Cloud-Trace-Context -header if set. Setting this
+    // to true will cause traces generated by this module to appear separately from other
+    // distributed work done by other services on behalf of the same income request. Setting
+    // this will also cause sampling decisions made by other distributed components to be
+    // ignored. This is useful for aggregating traces generated by different cloud platform
+    // projects.
     ignoreContextHeader: false
   }
 };