install commands for OpenShift

**Which component**:
The name (and version) of the affected component (controller or kubeseal)

> controller v0.28.0

**Is your feature request related to a problem? Please describe.**
A clear and concise description of what the problem is. Ex. I'm always frustrated when [...]

The Problem is that the default `controller.yaml` have `runAsUser` and `fsGroup` which do not work on OpenShift when `anyuid` is not allowed and no `fsGroup` is setuped.

You will get this message

> 72s Warning FailedCreate replicaset/sealed-secrets-controller-6979bd8b68 Error creating: pods "sealed-secrets-controller-6979bd8b68-" is forbidden: unable to validate against any security context constraint: [provider "anyuid": Forbidden: not usable by user or serviceaccount, provider "pipelines-scc": Forbidden: not usable by user or serviceaccount, provider "run-as-ray-user": Forbidden: not usable by user or serviceaccount, provider restricted-v2: .spec.securityContext.fsGroup: Invalid value: []int64{65534}: 65534 is not an allowed group, provider restricted-v2: .containers[0].runAsUser: Invalid value: 1001: must be in the ranges: [1001090000, 1001099999], provider "restricted": Forbidden: not usable by user or serviceaccount, provider "nonroot-v2": Forbidden: not usable by user or serviceaccount, provider "nonroot":

**Describe the solution you'd like**
A clear and concise description of what you want to happen.

The controller should run 😄

**Describe alternatives you've considered**
A clear and concise description of any alternative solutions or features you've considered.

I have solved the issue with this command.

```
curl -sSL https://github.com/bitnami-labs/sealed-secrets/releases/download/v0.28.0/controller.yaml |\
sed -E '/.*(fsGroup|runAsUser):.*/d' | oc apply -f -
```

In case the sealed-secrets controller should not run in `kube-system` can you add another `sed`, this requiers that the namespace `sealed-secrets` is created before the apply.

```
curl -sSL https://github.com/bitnami-labs/sealed-secrets/releases/download/v0.28.0/controller.yaml |\
sed 's/namespace: kube-system/namespace: sealed-secrets/g' | sed -E '/.*(fsGroup|runAsUser):.*/d' |\
oc apply -f -
```
**Additional context**
Add any other context or screenshots about the feature request here.

I think this could be also a solution for https://github.com/bitnami-labs/sealed-secrets/issues/1119

I think this info could be added into the README.

Start with the README and the controller.yaml installation manifest referenced in the issue. Document the curl and oc workflow that removes fsGroup and runAsUser for OpenShift, including the optional sealed-secrets namespace variant. Done means the README clearly explains both installation commands and the required namespace setup.

Written by the indexing model from the issue text.