Add docs for external-push scaler

Signed-off-by: Ahmed ElSayed <ahmels@microsoft.com>

content/docs/2.0/scalers/external-push.md

@@ -0,0 +1,49 @@
++++
+title = "External Push"
+layout = "scaler"
+availability = "v2.0+"
+maintainer = "Microsoft"
+description = "Scale applications based on an external push scaler."
+go_file = "external_scaler"
++++
+
+### Trigger Specification
+
+This specification describes the `external-push` trigger for an external push scaler.
+
+```yaml
+triggers:
+- type: external-push
+  metadata:
+    scalerAddress: external-scaler-service:8080
+    tlsCertFile: /path/to/tls/cert.pem # optional
+```
+
+**Parameter list:**
+
+- `scalerAddress`: "hostname:port" of the external push scaler implementing `ExternalScaler.StreamIsActive` in externalscaler.proto.
+- `tlsCertFile`: optional path for a certificate to use for the GRPC connection
+
+The entire metadata object is passed to the external scaler in `ScaledObjectRef.scalerMetadata`
+
+### Authentication Parameters
+
+Not supported.
+
+### Example
+
+```yaml
+apiVersion: keda.k8s.io/v1alpha1
+kind: ScaledObject
+metadata:
+  name: name
+  namespace: namespace
+spec:
+  scaleTargetRef:
+    deploymentName: keda-node
+  triggers:
+  - type: external-push
+    metadata:
+      scalerAddress: external-scaler-service:8080
+      tlsCertFile: /path/to/tls/cert.pem # optional
+```