environment variable AUTOVACUUM added

The postgressql autovacuum feature is by default active. The environment variable AUTOVACUUM allows you to switch off this feature during start of docker container.

README.md

@@ -31,10 +31,18 @@ Tiles that have already been rendered will be stored in `/var/lib/mod_tile`. To
 
 ## Performance tuning
 
+### THREADS
+
 The import and tile serving processes use 4 threads by default, but this number can be changed by setting the `THREADS` environment variable. For example:
 
     docker run -p 80:80 -e THREADS=24 -v openstreetmap-data:/var/lib/postgresql/10/main -d overv/openstreetmap-tile-server run
 
+### AUTOVACUUM
+
+The database use the autovacuum feature by default. This behavior can be changed with `AUTOVACUUM` environment variable. For example:
+
+    docker run -p 80:80 -e AUTOVACUUM=off -v openstreetmap-data:/var/lib/postgresql/10/main -d overv/openstreetmap-tile-server
+
 ## License
 
 ```