Update docker instructions to use --bind instead of -v. (#454)

* Update docker instructions to use --bind instead of -v.

This is the better and approved option now as `-v` will yield permission errors on some systems.

Signed-off-by: Martin Paul Eve <martin@martineve.com>

* Add requirement for directories to exist

Signed-off-by: Martin Paul Eve <martin@martineve.com>

README.md

@@ -86,14 +86,14 @@ It's recommended to create an alias for a convenient usage:
 
 ```bash
 alias whipper="docker run -ti --rm --device=/dev/cdrom \
-    -v ~/.config/whipper:/home/worker/.config/whipper \
-    -v ${PWD}/output:/output \
+    --mount type=bind,source=~/.config/whipper,target=/home/worker/.config/whipper \
+    --mount type=bind,source=${PWD}/output,target=/output \
     whipperteam/whipper"
 ```
 
 You should put this e.g. into your `.bash_aliases`. Also keep in mind to substitute the path definitions to something that fits to your needs (e.g. replace `… -v ${PWD}/output:/output …` with `… -v ${HOME}/ripped:/output \ …`).
 
-Make sure you create the configuration directory:
+Essentially, what this does is to map the /home/worker/.config/whipper and ${PWD}/output (or whatever other directory you specified) on your host system to locations inside the Docker container where the files can be written and read. These directories need to exist on your system before you can run the container:
 
 `mkdir -p ~/.config/whipper "${PWD}"/output`