Update mapcache_timedb README to mention available environment variables

mapcache_timedb/README.md

@@ -17,3 +17,23 @@ The container will run, wait for RabbitMQ events for new tiles or removed
 tiles, and update a local SQLite database with these times. See the `run.sh`
 for more details.
 
+## Options
+
+**Environment Variables**
+
+* `AMQPFIND_ARGS`: Extra arguments to send to amqpfind.py
+* `AMQPFIND_TOPIC`: The RabbitMQ topic to listen to for amqpfind.py
+* `G2G_PRODUCTS`: What products to expect and process. Defaults to all channel
+  products and the `true_color` RGB product.
+* `SATELLITES`: Satellite platforms to include in processing. Defaults to
+  "g16 g17".
+* `INSTRUMENTS`: Instruments to process. Defaults to "abi".
+* `SECTORS`: Sectors to process. Defaults to "radf radc radm1 radm2".
+* `SEED`: Whether or not to pre-seed the cache when layers are added. Can be
+  "0" for off (the default) or anything else for True.
+* `SEED_OVERLAYS`: Whether or not to pre-seed the basemap, borders, and
+  graticules layers when this container starts. For an empty cache this can
+  take a couple minutes per layer. Defaults to True, can be turned off by
+  specifying as "0". This does not obey the `SEED_ARGS` environment variable
+  (see below).
+* `SEED_ARGS`: Extra arguments to pass to `mapcache_seed`. Defaults to "".