add linkerd multicluster install with helm task (kubernetes#731)

* add linkerd mc with helm

Signed-off-by: Tarun Pothulapati <tarunpothulapati@outlook.com>

* fix nits around case,spellings

Signed-off-by: Tarun Pothulapati <tarunpothulapati@outlook.com>

* update linkerd mc release name

Signed-off-by: Tarun Pothulapati <tarunpothulapati@outlook.com>

* use correct chart name

Signed-off-by: Tarun Pothulapati <tarunpothulapati@outlook.com>

* update doc with allow equivalent

Signed-off-by: Tarun Pothulapati <tarunpothulapati@outlook.com>

* rename createNamespace to installNamespace for consistency

Signed-off-by: Tarun Pothulapati <tarunpothulapati@outlook.com>

* fold helm mc install into install_multicluster.md

Signed-off-by: Tarun Pothulapati <tarunpothulapati@outlook.com>

Co-authored-by: Thomas Rampelberg <thomas@saunter.org>

linkerd.io/content/2/tasks/installing-multicluster.md

@@ -292,3 +292,76 @@ Make sure to verify that the cluster's have started up successfully by running
 ```bash
 linkerd check
 ```
+
+## Installing the multicluster control plane components through Helm
+
+Linkerd's multicluster components i.e Gateway and Service Mirror can
+be installed via Helm rather than the `linkerd multicluster install` command.
+
+This while not only allows advanced configuration, but also allows users
+to bundle the multicluster installation as part of their existing Helm based
+installation pipeline.
+
+### Adding Linkerd's Helm repository
+
+First, Let's add the Linkerd's Helm repository by running
+
+```bash
+# To add the repo for Linkerd2 stable releases:
+helm repo add linkerd https://helm.linkerd.io/stable
+```
+
+### Helm multicluster install procedure
+
+By default, both the multicluster components i.e Service Mirror and Gateway are
+installed when no toggle values are added.
+
+```bash
+helm install linkerd2-multicluster linkerd/linkerd2-multicluster
+```
+
+The chart values will be picked from the chart's `values.yaml` file.
+
+You can override the values in that file by providing your own `values.yaml`
+file passed with a `-f` option, or overriding specific values using the family of
+`--set` flags.
+
+Full set of configuration options can be found [here](https://github.com/linkerd/linkerd2/tree/master/charts/linkerd2-multicluster#configuration)
+
+The installation can be verified by running
+
+```bash
+linkerd check --multicluster
+```
+
+Individual multicluster components can be enabled or disabled by setting
+`serviceMirror` and `gateway` respectively. By default, both of these
+values are true.
+
+### Installing access credentials
+
+For the source cluster to be able to access the target cluster's services, Access
+credentials have to be present in the target cluster. This can be done using the
+`linkerd multicluster allow` command through the CLI.
+
+The same functionality can also be done through Helm by disabling `gateway` and
+`serviceMirror` while submitting the remote service account name.
+
+```bash
+ helm install linkerd2-mc-soource linkerd/linkerd2-multicluster --set gateway=false --set serviceMirror=false --set remoteMirrorServiceAccountName=source --set installNamespace=false --kube-context target
+```
+
+{{< note >}}
+`installNamespace` should be disabled if the access credentials are
+being created in the same namespace as that of multicluster components to prevent
+failure due to namespace ownership conflict between the Helm releases.
+{{< /note >}}
+
+Each access credential is created as a separate Helm release providing clear separation.
+To revoke access to a particular source cluster, relevant helm release in the target
+cluster can be removed without effecting the mutlicluster components
+or other access credentials Helm releases.
+
+Now that the multicluster components are installed, operations like linking, etc
+can be performed by using the linkerd CLI's multicluster sub-command as per the
+[multicluster task](https://linkerd.io/2/features/multicluster).