Added more info about volumes (#2961)

* Added more info about volumes

Resolves #344

https://docs.docker.com/engine/tutorials/dockervolumes/

* Update dockervolumes.md

engine/tutorials/dockervolumes.md

@@ -37,7 +37,7 @@ Docker therefore *never* automatically deletes volumes when you remove
 a container, nor will it "garbage collect" volumes that are no longer
 referenced by a container.
 
-### Adding a data volume
+### Add a data volume
 
 You can add a data volume to a container using the `-v` flag with the
 `docker create` and `docker run` command. You can use the `-v` multiple times
@@ -54,7 +54,7 @@ This will create a new volume inside a container at `/webapp`.
 > You can also use the `VOLUME` instruction in a `Dockerfile` to add one or
 > more new volumes to any container created from that image.
 
-### Locating a volume
+### Locate a volume
 
 You can locate the volume on the host by utilizing the `docker inspect` command.
 
@@ -332,7 +332,19 @@ $ docker run --rm --volumes-from dbstore2 -v $(pwd):/backup ubuntu bash -c "cd /
 You can use the techniques above to automate backup, migration and
 restore testing using your preferred tools.
 
-## Removing volumes
+## List all volumes
+
+You can list all existing volumes using `docker volume ls`.
+
+```bash
+$ docker volume ls
+DRIVER              VOLUME NAME
+local               ec75c47aa8b8c61fdabcf37f89dad44266841b99dc4b48261a4757e70357ec06
+local               f73e499de345187639cdf3c865d97f241216c2382fe5fa67555c64f258892128
+local               tmp_data
+```
+
+## Remove volumes
 
 A Docker data volume persists after a container is deleted. You can create named
 or anonymous volumes. Named volumes have a specific source form outside the
@@ -347,6 +359,14 @@ $ docker run --rm -v /foo -v awesome:/bar busybox top
 This command creates an anonymous `/foo` volume. When the container is removed,
 the Docker Engine removes the `/foo` volume but not the `awesome` volume.
 
+To remove all unused volumes and free up space,
+
+```bash
+$ docker volume prune
+```
+
+it will remove all unused volumes which are not associated with any container.
+
 ## Important tips on using shared volumes
 
 Multiple containers can also share one or more data volumes. However, multiple