Add note regarding feature gate status

docs/network-policy.md

@@ -8,6 +8,13 @@ aimed towards developers to secure their apps and affects Pods within the
 Namespace in which the K8s NetworkPolicy is created.
 Rules belonging to ClusterNetworkPolicies are evaluated before any rule
 belonging to a K8s NetworkPolicy.
+**Note**: ClusterNetworkPolicy is currently in "Alpha" stage. In order to
+enable them, edit the Controller configuration in the `antrea` ConfigMap
+as follows:
+```yaml
+   antrea-controller.conf: |
+      ClusterNetworkPolicy: true
+```
 
 ## The ClusterNetworkPolicy resource
 
@@ -115,18 +122,18 @@ Hence, CNP take precedence over K8s NP.
 There are four kinds of selectors that can be specified in an ingress `from`
 section or egress `to` section:
 
-**podSelector**: This selects particular Pods from all Namespaces as `sources`,
-if set in `ingress` section, or as `destinations`, if set in `egress` section.
+**podSelector**: This selects particular Pods from all Namespaces as "sources",
+if set in `ingress` section, or as "destinations", if set in `egress` section.
 
 **namespaceSelector**: This selects particular Namespaces for which all Pods are
-grouped as `ingress` `sources` or `egress` `destinations`.
+grouped as `ingress` "sources" or `egress` "destinations".
 
 **podSelector** and **namespaceSelector**:  A single to/from entry that specifies
 both namespaceSelector and podSelector selects particular Pods within
 particular Namespaces. 
 
-**ipBlock**: This selects particular IP CIDR ranges to allow as `ingress` `sources`
-or `egress` `destinations`. These should be cluster-external IPs, since Pod IPs are
+**ipBlock**: This selects particular IP CIDR ranges to allow as `ingress` "sources"
+or `egress` "destinations". These should be cluster-external IPs, since Pod IPs are
 ephemeral and unpredictable.
 
 ## Key differences from K8s NetworkPolicy