Add some instructions for creating an OAuth 2.0 application

docs/create-an-oauth-application.md

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+# Create an OAuth 2.0 application
+
+Flickypedia is an OAuth 2.0 application on Wikimedia.
+
+If you want to create new API keys (either to run your own instance of Flickypedia or for development work):
+
+1.  Read [OAuth/For Developers][for_developers] in the MediaWiki docs.
+
+2.  Open the Special:OAuthConsumerRegistration/propose form:
+
+    *   [in the live Wikimedia Commons environment](https://meta.wikimedia.org/wiki/Special:OAuthConsumerRegistration/propose)
+    *   [in the beta Wikimedia Commons environment](https://meta.wikimedia.beta.wmflabs.org/wiki/Special:OAuthConsumerRegistration/propose)
+
+3.  Click "Propose an OAuth 2.0 client".
+
+4.  Fill in the Application name, Consumer version, Application description, and so on.
+
+    In the list of applicable grants, select the following:
+
+    *   Create, edit, and move pages
+    *   Upload new files
+
+You will get a client ID and client secret.
+Make sure you write these down -- you'll need them later!
+
+[for_developers]: https://www.mediawiki.org/wiki/OAuth/For_Developers