Explain helm chart install on restricted environments (bitnami-labs#938)

Users within an enterprise might face very restrictive installation environments for Sealed Secrets.

This change adds documentation specific on how to install from the Helm Chart on one of the most restrictive cases out there: Access to a single namespace and service account only, no cluster access at all.

README.md

@@ -304,6 +304,49 @@ Alternatively, you can set `fullnameOverride` when installing the chart to overr
 helm install sealed-secrets -n kube-system --set-string fullnameOverride=sealed-secrets-controller sealed-secrets/sealed-secrets 
 ```
 
+##### Helm Chart on a restricted environment
+
+In some companies you might be given access only to a single namespace, not a full cluster.
+
+One of the most restrictive environments you can encounter is:
+- A `namespace` was allocated to you with some `service account`.
+- You do not have access to the rest of the cluster, not even cluster CRDs.
+- You may not even be able to create further service accounts or roles in your namespace.
+- You are required to include resource limits in all your deployments.
+
+Even with these restrictions you can still install the sealed secrets Helm Chart, there is only one pre-requisite:
+- *The cluster must already have the sealed secrets CRDs installed*.
+
+Once your admins installed the CRDs, if they were not there already, you can install the chart by preparing a YAML config file such as this:
+
+```shell
+serviceAccount:
+  create: false
+  name: {allocated-service-account}
+rbac:
+  create: false
+  clusterRole: false
+resources: 
+  limits: 
+    cpu: 150m
+    memory: 256Mi
+```
+
+Note that:
+- No service accounts are created, instead the one allocated to you will be used.
+  - `{allocated-service-account}` is the name of the `service account` you were allocated on the cluster.
+- No RBAC roles are created neither in the namespace nor the cluster.
+- Resource limits must be especified.
+  - The limits are samples that shoudl work, but you might want to review them in your particular setup.
+
+Once that file is ready, if you named it `config.yaml` you now can install the sealed secrets Helm Chart like this: 
+
+```shell
+helm install sealed-secrets -n {allocated-namespace} sealed-secrets/sealed-secrets --skip-crds -f config.yaml
+```
+
+Where `{allocated-namespace}` is the name of the `namespace` you were allocated in the cluster.
+
 ### Homebrew
 
 The `kubeseal` client is also available on [homebrew](https://formulae.brew.sh/formula/kubeseal):