Add Docker instructions in README (resolves #4)

README.md

@@ -42,4 +42,24 @@ The first command will backup your saved posts/comments to a file called folder_
 
 Each post will have its top-level comments saved, as well as each of their immediate child comments (but no further).
 
-Linked media files (images, videos etc.) will be saved locally where possible, though imgur is currently not well supported in all cases.
+Linked media files (images, videos etc.) will be saved locally where possible, though imgur is currently not well supported in all cases.
+
+## Use with Docker
+
+Rather than installing dependencies locally, you can use docker to create a local image and use that instead. First build the image:
+
+```bash
+$ docker build -t redditsave .
+```
+
+Then run reddit-save within a container created from this image:
+
+```bash
+$ docker run \
+-e REDDIT_USERNAME=spez \
+-e REDDIT_PASSWORD="myredditpassword123" \
+-e REDDIT_CLIENT_ID="sadsU7-zfX" \
+-e REDDIT_SECRET="687DDJSS&999d-hdkjK8h" \
+-v /Local/location/to/save/in:/opt/app/archive \
+redditsave saved
+```