Release cleanup (#27)

* Update changelog for release. Add missing documentation.

* switch to preview.4

* Updating LICENSE.md to include required header.

* Reverting accidental addition.

LICENSE.md

+com.unity.perception copyright © 2020 Unity Technologies ApS
+
                                  Apache License
                            Version 2.0, January 2004
                         http://www.apache.org/licenses/

com.unity.perception/CHANGELOG.md

 
 ### Security
 
+## [0.1.0] - 2020-06-24
+### This is the first release of the _Perception_ package

com.unity.perception/LICENSE.md

+com.unity.perception copyright © 2020 Unity Technologies ApS
+
                                  Apache License
                            Version 2.0, January 2004
                         http://www.apache.org/licenses/

com.unity.perception/Runtime/GroundTruth/DatasetCapture.cs

         /// Report a metric not associated with any sensor or annotation.
         /// </summary>
         /// <param name="metricDefinition">The metric definition of the metric being reported</param>
+        /// <returns>An <see cref="AsyncMetric"/> which should be used to report the metric values, potentially in a later frame</returns>
         public static AsyncMetric ReportMetricAsync(MetricDefinition metricDefinition) => SimulationState.CreateAsyncMetric(metricDefinition);
 
         /// <summary>

com.unity.perception/Runtime/GroundTruth/Ego.cs

         /// </summary>
         public string Description;
         EgoHandle m_EgoHandle;
-
+        
+        /// <summary>
+        /// The EgoHandle registered with the SimulationManager at runtime.
+        /// </summary>
         public EgoHandle EgoHandle
         {
             get

com.unity.perception/package.json

     "displayName": "Perception", 
     "name": "com.unity.perception", 
     "unity": "2019.3", 
-    "version": "0.1.0-preview.3"
+    "version": "0.1.0-preview.4"
 }