Create OAuth.md

Author: tschie

OAuth.md

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+Some samples require you to configure your own portal for authentication. This page describes the OAuth pattern.
+
+### Register an App
+Login to your [ArcGIS Developers site](http://developers.arcgis.com) account and [register](https://developers.arcgis.com/applications/#/new/) an app.
+
+### Portal URL
+For an app registered and hosted on ArcGIS.com, use `http://www.arcgis.com/` as the portal URL.
+
+### Client ID
+Once registered, select the Authentication tab to get your **Client ID**.
+
+### Redirect URL
+In the Authentication tab, go to the Redirect URIs section and add `urn:ietf:wg:oauth:2.0:oob`.