Document how to login to the Marketplace from the Market app

This fixes #619.

modules/user_manual/pages/apps/market.adoc

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+= The Market App
+:keywords: ownCloud Marketplace, ownCloud Market, apps
+:description: Here you will find out all you need to know about working with ownCloud's Market app.
+
+== Log in to the Marketplace From the Market App
+
+To log in to {oc-marketplace-url}[the ownCloud Marketplace] from the Market app, open the Market app by clicking on the Market app icon in the top-level ownCloud navigation menu, which you can see above.
+
+image:apps/market/top-level-navigation-bar-highlighting-the-market-app.png[The Market app in the ownCloud top-level navigation menu]
+
+Once on the Market app, click btn:[LOGIN], located at the bottom of the left-hand side navigation menu.
+
+image:apps/market/login-step-1.png[Log in to the Marketplace from the Market App, step 1]
+
+You are next asked to grant the Market app access to your Marketplace account.
+Enter your Marketplace username and password and click btn:[LOGIN].
+
+image:apps/market/login-step-2.png[Log in to the Marketplace from the Market App, step 2]
+
+If your credentials authenticated correctly, you are next asked to confirm your request to grant the Market app access to your Marketplace account. 
+To give this confirmation, click btn:[GRANT ACCESS].
+
+image:apps/market/login-step-3.png[Log in to the Marketplace from the Market App, step 3]
+
+Once you've done this, you are redirected back to the Market app in your ownCloud installation.
+The original "_LOGIN_" button is now greyed out and labelled "_LOGGED IN_".
+
+image:apps/market/login-step-4.png[Log in to the Marketplace from the Market App, step 4]