Create installing_apps.rst

I suggest to amend the documentation and add the following to the apps.owncloud.com website as well. Since this is a new section and describes issues under active discussion I am putting this up as an issue, inviting comments and corrections as well as instructions from the maintainers as to where exactly this might be placed in the documentation.
Update installing_apps.rst
Update installing_apps.rst
Update installing_apps.rst
Update installing_apps.rst
Update installing_apps.rst
Update installing_apps.rst

typos corrected, app is lowercase, Apps menu is uppercase in keeping with the interface conventions - or so I thought?
Update installing_apps.rst
Merge pull request #474 from menelic/master

Create installing_apps.rst