Add a little bit of commentary on ArcTool, a=chris

diff --git a/src/executables/ArcTool.php b/src/executables/ArcTool.php
index 44375e0ab..c380177c6 100755
--- a/src/executables/ArcTool.php
+++ b/src/executables/ArcTool.php
@@ -539,6 +539,8 @@ class ArcTool implements CrawlConstants
             exit();
         }
         $shards = glob($path."/posting_doc_shards/index*");
+        $num_shards = count($shards);
+        echo "Total number of shards to reindex is: $num_shards";
         if (is_array($shards)) {
             $dbms_manager = C\NS_DATASOURCES . ucfirst(C\DBMS) . "Manager";
             $db = new $dbms_manager();
@@ -556,7 +558,7 @@ class ArcTool implements CrawlConstants
                 }
                 for ($i = $start_shard; $i < $max_generation + 1; $i++) {
                     $shard_name = $path."/posting_doc_shards/index$i";
-                    echo "\nShard $i\n";
+                    echo "\nShard $i of $num_shards\n";
                     $shard = new IndexShard($shard_name, $i,
                         C\NUM_DOCS_PER_GENERATION, true);
                     $dictionary->addShardDictionary($shard);