docs: add note on rolling back to the previous release (#5228)

While deving at a Microsoft Open Hack my group discovered this useful piece of information in this issue comment: https://github.com/helm/helm/issues/1796#issuecomment-311385728

We found it very useful for our Blue Green CD pipeline and thought others might find it useful as well.

Signed-off-by: Ethan Arrowood <ethan.arrowood@gmail.com>
Signed-off-by: Matthew Fisher <matt.fisher@microsoft.com>
Signed-off-by: Kevin Labesse <kevin@labesse.me>

cmd/helm/rollback.go

@@ -31,7 +31,8 @@ This command rolls back a release to a previous revision.
 
 The first argument of the rollback command is the name of a release, and the
 second is a revision (version) number. To see revision numbers, run
-'helm history RELEASE'.
+'helm history RELEASE'. If you'd like to rollback to the previous release use
+'helm rollback [RELEASE] 0'.
 `
 
 type rollbackCmd struct {

docs/helm/helm_rollback.md

@@ -9,7 +9,8 @@ This command rolls back a release to a previous revision.
 
 The first argument of the rollback command is the name of a release, and the
 second is a revision (version) number. To see revision numbers, run
-'helm history RELEASE'.
+'helm history RELEASE'. If you'd like to rollback to the previous release use
+'helm rollback [RELEASE] 0'.
 
 
 ```
@@ -51,4 +52,4 @@ helm rollback [flags] [RELEASE] [REVISION]
 
 * [helm](helm.md)	 - The Helm package manager for Kubernetes.
 
-###### Auto generated by spf13/cobra on 10-Aug-2018
+###### Auto generated by spf13/cobra on 29-Jan-2019