move user docs to their new home

docs/user-guide/identifiers.md

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+<h1>*** PLEASE NOTE: This document applies to the HEAD of the source
+tree only. If you are using a released version of Kubernetes, you almost
+certainly want the docs that go with that version.</h1>
+
+<strong>Documentation for specific releases can be found at
+[releases.k8s.io](http://releases.k8s.io).</strong>
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+# Identifiers
+All objects in the Kubernetes REST API are unambiguously identified by a Name and a UID.
+
+For non-unique user-provided attributes, Kubernetes provides [labels](labels.md) and [annotations](annotations.md).
+
+## Names
+Names are generally client-provided.  Only one object of a given kind can have a given name at a time (i.e., they are spatially unique).  But if you delete an object, you can make a new object with the same name.  Names are the used to refer to an object in a resource URL, such as `/api/v1/pods/some-name`.   By convention, the names of Kubernetes resources should be up to maximum length of 253 characters and consist of lower case alphanumeric characters, `-`, and `.`, but certain resources have more specific restrictions.  See the [identifiers design doc](design/identifiers.md) for the precise syntax rules for names.
+
+## UIDs
+UID are generated by Kubernetes.  Every object created over the whole lifetime of a Kubernetes cluster has a distinct UID (i.e., they are spatially and temporally unique).
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