doc: alpha-features.md

doc/alpha-features.md

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+# Alpha Features
+
+Tracking document for alpha features of Kubernetes that etcd operators makes use of.
+
+We track these alpha features as their behavior may change or be deprecated between Kubernetes versions.
+Therefore clusters that use these features need to keep track of any potential changes in upstream releases.
+See the upstream [api versioning documentation](https://github.com/kubernetes/community/blob/master/contributors/devel/api_changes.md#alpha-beta-and-stable-versions) for more information.
+
+
+### TolerateUnreadyEndpointsAnnotation
+
+Used by the etcd client and peer service object.
+
+This alpha annotation will retain the endpoints even if the etcd pod isn't ready.
+This feature is always enabled in endpoint controller in k8s even it is alpha.
+
+References:
+- https://github.com/coreos/etcd-operator/issues/622
+- https://github.com/coreos/etcd-operator/issues/1257