Add some information to PerformanceManager documentation

Change-Id: I79e1153db46c9d12b6bf774914a46b095eeb3ef4
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2025947Reviewed-by: Sigurður Ásgeirsson <siggi@chromium.org>
Commit-Queue: Oliver Li <olivierli@chromium.org>
Auto-Submit: Oliver Li <olivierli@chromium.org>
Cr-Commit-Position: refs/heads/master@{#738219}

components/performance_manager/README.md

@@ -4,6 +4,15 @@ The Performance Manager centralizes policy for data-driven resource management
 and planning. Central to this is the [graph](graph/graph_impl.h) which is
 comprised of nodes that reflect the coarse structure of a browser at the
 level of [pages](graph/page_node_impl.h), [frames](graph/frame_node_impl.h)
-[processes](graph/process_node_impl.h) and so forth. Each node is adorned with
-relationships and properties sufficient to allow performance management policy
-to reason about such things as resource usage and prioritization.
+[processes](graph/process_node_impl.h), [workers](graph/worker_node_impl.h) and so forth.
+
+Each node is adorned with relationships and properties sufficient to allow performance management policy
+to reason about such things as resource usage and prioritization. Usually properties are added directly
+to nodes if they would be used by multiple clients. Otherwise, it's preferred to 
+add them as private impl details via [NodeAttachedData](graph/node_attached_data_impl.h). See
+[ProcessPriorityAggregator](/chrome/browser/performance_manager/decorators/process_priority_aggregator.cc) as an example of
+such a use-case. It requires some per-process-node intermediate data which is stored as NodeAttachedData.
+
+Nodes are nothing but data storage and contain no logic. Properties that need
+computation to be maintained are maintained via [decorators](decorators/) or "aggregators"
+which are both observers and mutators of the graph.