Add the README meta file

Assets/ScriptableRenderLoop/HDRenderLoop/Sky/README.txt.meta

+fileFormatVersion: 2
+guid: 4eead1bf6efbd0a4e852367accddc070
+timeCreated: 1480334378
+licenseType: Pro
+TextScriptImporter:
+  userData: 
+  assetBundleName: 
+  assetBundleVariant: