Add screenshot, adjust makefile, info.xml and README

Makefile

@@ -78,6 +78,7 @@ sign:
 	--exclude=$(app_name)/js/vendor/jquery-timepicker/legacy_1.2.6 \
 	--exclude=$(app_name)/js/vendor/jquery-timepicker/tests \
 	--exclude=$(app_name)/js/vendor/jquery-timepicker/index.html \
+	--exclude=$(app_name)/screenshots \
 	--exclude=$(app_name)/timezones/INFO.md
 	chown -R www-data $(signed_dir)
 	sudo -u www-data php ../../occ integrity:sign-app --privateKey=$(build_dir)/app-signing/tasks.key --certificate=$(build_dir)/app-signing/tasks.crt --path=$(signed_dir)/$(app_name)

README.md

@@ -8,11 +8,11 @@ Tasks app for Nextcloud.
 
  * **Using git:** In your `nextcloud/apps/` directory, run `git clone https://github.com/nextcloud/tasks/`. Then go to the Apps manager tab in your Nextcloud web interface, and enable the Tasks app.
  * **Using the zip archive:** Download the latest [zip archive](https://github.com/nextcloud/tasks/archive/master.zip), extract the `tasks-master` directory inside your `nextcloud/apps/` directory, and rename it to `tasks`. For stable releases, you can also download one of the [releases](https://github.com/nextcloud/tasks/releases) tar.gz archives. Then go to the Apps manager tab in your Nextcloud web interface, and enable the Tasks app.
- 
+
 ### Usage/Features
 
 Once enabled, a new `Tasks` menu will appear in your Nextcloud apps menu. From there you can **add and delete tasks, edit their title, description,  start and due dates, reminder times, mark them as important, and add comments on them**. Tasks can be **shared** between users. Tasks can be **synchronized using CalDav** (each task list is linked to an Nextcloud calendar, to sync it to your local client - Thunderbird, Evolution, KDE Kontact, iCal... - just add the calendar as a remote calendar in you client). You can **download your tasks as ICS** files using the download button for each calendar.
 
 ### Screenshot
 
-![tasks](https://cloud.githubusercontent.com/assets/2496460/9783697/ab337634-57a6-11e5-9633-b8f950b1e28e.png)
+![tasks](https://raw.githubusercontent.com/nextcloud/tasks/master/screenshots/tasks-1.png)

appinfo/info.xml

@@ -9,6 +9,7 @@
     <author mail="[email protected]">Raimund Schlüßler</author>
     <version>0.9.4</version>
     <ocsid>164356</ocsid>
+    <screenshot>https://raw.githubusercontent.com/nextcloud/tasks/master/screenshots/tasks-1.png</screenshot>
     <namespace>Tasks</namespace>
 	<category>organization</category>
     <dependencies>