Reporting: Clarify "pending" reports, add README.md.

net/reporting/reporting_cache.h

Index: net/reporting/reporting_cache.h
diff --git a/net/reporting/reporting_cache.h b/net/reporting/reporting_cache.h
index 3bc60ad1e0949f5ca16bb6fca7c2274d6561b12d..fdd19c29eeb88a3cc4dcae767cf13806812c63a2 100644
--- a/net/reporting/reporting_cache.h
+++ b/net/reporting/reporting_cache.h
@@ -31,6 +31,11 @@ struct ReportingReport;
 // This corresponds roughly to the "Reporting cache" in the spec, except that
 // endpoints and clients are stored in a more structurally-convenient way, and
 // endpoint failures/retry-after are tracked in ReportingEndpointManager.
+//
+// The cache implementation has the notion of "pending" reports. These are
+// reports that are part of an active delivery attempt, so they won't be
+// actually deallocated. Any attempt to remove a pending report wil mark it
+// "doomed", which will cause it to be deallocated once it is no longer pending.
 class NET_EXPORT ReportingCache {
  public:
   ReportingCache();