Add documentation for Cassandra scaler (kedacore#560)

Signed-off-by: nilayasiktoprak <nilayasiktoprak@gmail.com>

content/docs/2.5/scalers/cassandra.md

@@ -0,0 +1,90 @@
++++
+title = "Cassandra"
+availability = "v2.5+"
+maintainer = "Community"
+description = "Scale applications based on Cassandra query results."
+layout = "scaler"
+go_file = "cassandra_scaler"
++++
+
+### Trigger Specification
+
+This specification describes the `cassandra` trigger that scales based on the outputs of a Cassandra query.
+
+```yaml
+triggers:
+  - type: cassandra
+    metadata:
+      username: "cassandra"
+      port: "9042"
+      clusterIPAddress: "cassandra.default"
+      consistency: "Quorum"
+      protocolVersion: "4"
+      keyspace: "test_keyspace"
+      query: "SELECT COUNT(*) FROM test_keyspace.test_table;"
+      targetQueryValue: "1"
+      metricName: "test_keyspace"
+```
+
+**Parameter list:**
+
+- `username` - The username credential for connecting to the Cassandra instance.
+- `port` - The port number of the Cassandra instance. (Optional, Can be set either here or in `clusterIPAddress`)
+- `clusterIPAddress` - The IP address or the host name of the Cassandra instance.
+- `consistency` - Configuration for a session or per individual read operation. (Values: `LOCAL_ONE`, `LOCAL_QUORUM`, `EACH_QUORUM`, `LOCAL_SERIAL`, `ONE`, `TWO`, `THREE`, `QUORUM`, `SERIAL`, `ALL`, Default: `ONE`, Optional)
+- `protocolVersion` - CQL Binary Protocol. (Default: `4`, Optional)
+- `keyspace` - The name of the keyspace used in Cassandra.
+- `query` - A Cassandra query that should return single numeric value.
+- `targetQueryValue` - The threshold value that is provided by the user and used as `targetAverageValue` in the Horizontal Pod Autoscaler (HPA).
+- `metricName` - Name to assign to the metric. (Default: `s<X>-cassandra-<KEYSPACE>`, Optional, In case of `metricName` is specified, it will be used to generate the `metricName` like this: `s<X>-cassandra-<METRICNAME>`, where `<X>` is the index of the trigger in a ScaledObject)
+
+### Authentication Parameters
+
+You can authenticate by using a password via `TriggerAuthentication` configuration.
+
+**Password Authentication:**
+
+- `password` - Password for configured user to login to the Cassandra instance.
+
+### Example
+
+```yaml
+apiVersion: v1
+kind: Secret
+metadata:
+  name: cassandra-secrets
+type: Opaque
+data:
+  cassandra_password: CASSANDRA_PASSWORD
+---
+apiVersion: keda.sh/v1alpha1
+kind: TriggerAuthentication
+metadata:
+  name: keda-trigger-auth-cassandra-secret
+spec:
+  secretTargetRef:
+  - parameter: password
+    name: cassandra-secrets
+    key: cassandra_password
+---
+apiVersion: keda.sh/v1alpha1
+kind: ScaledObject
+metadata:
+  name: cassandra-scaledobject
+spec:
+  scaleTargetRef:
+    name: nginx-deployment
+  triggers:
+  - type: cassandra
+    metadata:
+      username: "cassandra"
+      port: "9042"
+      clusterIPAddress: "cassandra.default"
+      consistency: "Quorum"
+      protocolVersion: "4"
+      query: "SELECT COUNT(*) FROM test_keyspace.test_table;"
+      targetQueryValue: "1"
+      metricName: "test_keyspace"
+    authenticationRef:
+      name: keda-trigger-auth-cassandra-secret      
+```