Add eus channel info

Author: sdodson

modules/understanding-upgrade-channels.adoc

@@ -24,6 +24,7 @@ binary file for a specific version of {product-title} always installs that versi
 * `candidate-{product-version}`
 * `fast-{product-version}`
 * `stable-{product-version}`
+* `eus-4.6` (only available when running 4.6)
 
 [discrete]
 == candidate-{product-version} channel
@@ -49,7 +50,7 @@ builds are available for early access to features, but updating to or from night
 builds is neither recommended nor supported. Nightly builds are not available in
 any upgrade channel. You can reference the {product-title}
 ifdef::openshift-origin[]
-link:https://origin-release.apps.ci.l2s4.p1.openshiftapps.com/[release statuses] 
+link:https://origin-release.apps.ci.l2s4.p1.openshiftapps.com/[release statuses]
 endif::[]
 ifndef::openshift-origin[]
 link:https://openshift-release.apps.ci.l2s4.p1.openshiftapps.com/[release statuses]
@@ -82,6 +83,30 @@ and pre-production and production environments that participate in connected cus
 You can use the `stable-{product-version}` channel to upgrade from a previous minor version of
 {product-title}.
 
+[discrete]
+== eus-4.6 channel
+
+In addition to the stable channel, certain minor versions of {product-title} offer an
+Extended Update Support[https://access.redhat.com/support/policy/updates/openshift#ocp4_phases] (EUS).
+These EUS versions extend the maintenance phase for customers with Premium Subscriptions
+to 14 months. {product-title} 4.6 is currently the only minor version with EUS.
+
+Although there is no difference between stable-4.6 and eus-4.6
+channels until {product-title} 4.6 transitions to EUS the phase, you can switch
+to the EUS channel as soon as it becomes available. When {product-title} 4.6
+transitions to the EUS phase of its lifecycle the stable-4.6 channel will
+no longer receive subsequent z-stream updates. After you upgrade to a version that
+is exclusive to the EUS channel, that cluster will no longer be eligible for minor
+version upgrades until upgrades to the next EUS version become available. The next
+planned EUS version is to be 4.10 and the upgrade to that version will require a
+serial set of version upgrades, such as from 4.6 to 4.7 to 4.8 to 4.9 to 4.10.
+
+Additionally, you may only switch to the EUS channel when your cluster is running
+a supported version of {product-title} 4.6.
+
+Finally, if you install of a 4.6 version that is exclusive to EUS, you will
+similarly not be able to upgrade to a later minor version until upgrades are provided to 4.10.
+
 [discrete]
 == Upgrade version paths