[docs] Storage documantation

doc/storage.md

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+# Storage
+
+The default storage for _**Limitador**_ _limits counter_ is in memory, which there's no configuration needed.
+In order to configure a Redis data structure store, currently there are 2 alternatives:
+
+* Redis
+* Redis Cached
+
+For any of those, one should store the URL of the Redis service, inside a K8s opaque 
+[Secret](https://kubernetes.io/docs/concepts/configuration/secret/).
+
+```yaml
+apiVersion: v1
+kind: Secret
+metadata:
+  name: redisconfig
+stringData:
+  URL: redis://127.0.0.1/a # Redis URL of its running instance
+type: Opaque
+```
+
+It's also required to setup `Spec.Storage`
+
+## Redis
+
+```yaml
+apiVersion: limitador.kuadrant.io/v1alpha1
+kind: Limitador
+metadata:
+  name: limitador-sample
+spec:
+  storage:
+    redis:
+      configSecretRef: # The secret reference storing the URL for Redis
+        name: redisconfig
+        namespace: default # optional
+  limits:
+    - conditions: ["get-toy == yes"]
+      max_value: 2
+      namespace: toystore-app
+      seconds: 30
+      variables: []
+```
+
+## Redis Cached
+
+### Options
+
+| Option       | Description                                                       |
+|--------------|-------------------------------------------------------------------|
+| ttl          | TTL for cached counters in milliseconds [default: 5000]           |
+| ratio        | Ratio to apply to the TTL from Redis on cached counters [default: |
+| flush-period | Flushing period for counters in milliseconds [default: 1000]      |
+ | max-cached   | Maximum amount of counters cached [default: 10000]                |
+
+
+```yaml
+apiVersion: limitador.kuadrant.io/v1alpha1
+kind: Limitador
+metadata:
+  name: limitador-sample
+spec:
+  storage:
+    redis-cached:
+      configSecretRef: # The secret reference storing the URL for Redis
+        name: redisconfig
+        namespace: default # optional
+     options: # Every option is optional
+        ttl: 1000      
+       
+  limits:
+    - conditions: ["get-toy == yes"]
+      max_value: 2
+      namespace: toystore-app
+      seconds: 30
+      variables: []
+```