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.
Ah ok, misread the above sentence. Maybe you could add the issue tracker in addition to the comments section.