doc/data_flow* diagrams added

doc/data_flow_realtime.dot

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+digraph DruidDataFlow {
+        graph [bgcolor="#fffef5", clusterrank=global, rankdir=TB];
+        node [color="#0a0701", fillcolor="#fdf4c6", fontname=Helvetica, shape=box, style=filled, label="\N"];
+        edge [color="#377d18"];
+
+  1 [label = "REST query"];
+  2 [label = "realtime_data_src"];
+  4 [label = "indexed segments\nblob_store (S3, HDFS)"];
+  5 [label = "Realtime.working_set"];
+  6 [label = "Realtime.spill_disk"];
+  7 [label = "HDFS_data_src"];
+  8 [label = "external_data_src"];
+  9 [label = "Realtime.local_disk"];
+ 10 [label = "metadata_store (mysql)"];
+ 11 [label = "HadoopDruidIndexer.working_set"];
+ 12 [label = "IndexerService.working_set"];
+
+2 -> 5 [label = "Realtime.Firehose"];
+5 -> 6 [label = "Realtime.Firehose"];
+
+5 -> 9 [label = "Realtime.Indexer"];
+6 -> 9 [label = "Realtime.Indexer"];
+5 -> 10 [label = "Realtime.MetadataUpdater"];
+
+7 -> 11 [label = "HadoopDruidIndexer"];
+11 -> 4 [label = "HadoopDruidIndexer"];
+11 -> 10 [label = "HadoopDruidIndexer"];
+
+8 -> 12 [label = "IndexerService"];
+12 -> 4 [label = "IndexerService"];
+12 -> 10 [label = "IndexerService"];
+
+
+9 -> 4 [label = "Realtime.segmentPusher"];
+
+5 -> 1  [label = " results "];
+
+1 -> 5 [label = " query "];
+}

doc/data_flow_simple.dot

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+digraph DruidDataFlow {
+        graph [bgcolor="#fffef5", clusterrank=global, rankdir=TB];
+        node [color="#0a0701", fillcolor="#fdf4c6", fontname=Helvetica, shape=box, style=filled, label="\N"];
+        edge [color="#377d18"];
+
+  1 [label = "REST query"];
+ 10 [label = "Broker.query_cache"];
+  2 [label = "realtime_data_src"];
+  3 [label = "Compute.disk_cache"];
+  4 [label = "indexed segments\nblob_store (S3, HDFS)"];
+  5 [label = "Realtime.working_set"];
+  6 [label = "Compute.working_set"];
+
+2 -> 5 [label = "Realtime.Firehose"];
+5 -> 4 [label = "Realtime.segmentPusher"];
+
+4 -> 3 [label = "Compute.load"];
+3 -> 6 [label = "Compute.map"];
+
+5 -> 10  [label = " results "];
+6 -> 10  [label = " results "];
+10 -> 1 [label = " results "];
+
+10 -> 5  [label = " query "];
+10 -> 6  [label = " query "];
+1 -> 10 [label = " query "];
+}