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 a partial solution to https://github.com/owncloud/documentation/issues/473 inviting comments and corrections as well as instructions from the maintainers as to where exactly this might be placed in the documentation.
Exactly, its confusing and annoying for both parties. Ideally we could turn off the comments section and refer people to the issue tracker instead. In fact I think that the issue tracker should be required for every uploaded app, but the appstore is going to be changed anyways (@MorrisJobke works on the mozilla app store).