Add kubernetes provider documentation

docs/provider-kubernetes.md

@@ -0,0 +1,58 @@
+External Secrets Operator allows to retrieve in-cluster secrets or from a remote Kubernetes Cluster.
+
+### Authentication
+
+It's possible to authenticate against the Kubernetes API using client certificates, a bearer token or a service account (not implemented yet). The operator enforces that exactly one authentication method is used.
+
+## Example
+
+### SecretStore
+
+The `Server` section specifies the url of the Kubernetes API and the location to fetch the CA. The `auth` section indicates the type of authentication to use, `cert`, `token` or `serviceAccount` and includes the path to fetch the certificates or the token.
+
+
+```
+apiVersion: external-secrets.io/v1alpha1
+kind: SecretStore
+metadata:
+  name: example
+spec:
+  provider:
+      kubernetes:  
+        server: 
+          url:  https://127.0.0.1:36473
+          caProvider: 
+            type: Secret
+            name : kind-cluster-secrets
+            key: ca
+        auth:
+          cert:
+            clientCert: 
+                name: kind-cluster-secrets
+                key: certificate
+            clientKey: 
+                name: kind-cluster-secrets
+                key: key
+```
+        
+### ExternalSecret
+
+```
+apiVersion: external-secrets.io/v1alpha1
+kind: ExternalSecret
+metadata:
+  name: example
+spec:
+  refreshInterval: 1h           
+  secretStoreRef:
+    kind: SecretStore
+    name: example               # name of the SecretStore (or kind specified)
+  target:
+    name: secret-to-be-created  # name of the k8s Secret to be created
+    creationPolicy: Owner
+  data:
+  - secretKey: extra #
+    remoteRef:
+      key: secret-example
+      property: extra
+```

hack/api-docs/mkdocs.yml

@@ -63,6 +63,7 @@ nav:
       - Oracle Vault: provider-oracle-vault.md
     - Webhook: provider-webhook.md
     - Fake: provider-fake.md
+    - Kubernetes: provider-kubernetes.md
   - Examples:
     - FluxCD: examples-gitops-using-fluxcd.md
     - Anchore Engine: examples-anchore-engine-credentials.md