better docker documentation

docs/index.md

@@ -34,6 +34,8 @@ bbot --help
 
 Docker images are provided, along with helper script `bbot-docker.sh` to persist your scan data.
 
+Scans are output to `~/.bbot/scans` (the usual place for BBOT scan data).
+
 ```bash
 # bleeding edge (dev)
 docker run -it blacklanternsecurity/bbot --help
@@ -46,6 +48,16 @@ git clone https://github.com/blacklanternsecurity/bbot && cd bbot
 ./bbot-docker.sh --help
 ```
 
+Note: If you need to pass in a custom preset, you can do so by mapping the preset into the container:
+
+```bash
+# use the preset `my_preset.yml` from the current directory
+docker run --rm -it \
+  -v "$HOME/.bbot/scans:/root/.bbot/scans" \
+  -v "$PWD/my_preset.yml:/my_preset.yml" \
+  blacklanternsecurity/bbot -p /my_preset.yml
+```
+
 ## Example Commands
 
 Below are some examples of common scans.