Bug 1212319 - Add docs for Worker markers, r=jsantell

devtools/client/performance/docs/markers.md

@@ -144,3 +144,18 @@ A marker generated via `console.timeStamp(label)`.
 ## Parse HTML
 
 ## Parse XML
+
+## Worker
+
+Emitted whenever there's an operation dealing with Workers (any kind of worker,
+Web Workers, Service Workers etc.). Currently there are 4 types of operations
+being tracked: serializing/deserializing data on the main thread, and also
+serializing/deserializing data off the main thread.
+
+* ProfileTimelineWorkerOperationType operationType - the type of operation
+  being done by the Worker or the main thread when dealing with workers.
+  Can be one of the following enums defined in ProfileTimelineMarker.webidl
+  * "serializeDataOffMainThread"
+  * "serializeDataOnMainThread"
+  * "deserializeDataOffMainThread"
+  * "deserializeDataOnMainThread"