Only uses one worker for Celery

README.md

@@ -1,5 +1,11 @@
 
 
-Tile generation uses Celery, to launch a worker, run:
+We use Celery for heavy computations (tile generation, and optionally
+assembling panoramas).  To launch the celery workers, run:
 
-    celery -A ztulec.celery worker --loglevel=info
+    celery -c 1 -A ztulec.celery worker --loglevel=info
+
+Notice that we limit the number of worker to 1 (by default, celery launches
+as many workers as there are CPU cores).  This because assembling panoramas
+is already multi-threaded and uses a lot of RAM, so there is no point in
+running multiple assembling in parallel.