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.
I know that this is a controversial issue and I am aware of the troubles you have with the news app, of which I'm a fan. Part of the reason I wrote this is my wrangling with the false positive the app code checker used to throw up. I agree that there are many issues with that code checker, but since users might come across info on that when they run into issues I feel some info on that should be in there. Maybe it could read "the app has been rejected by the internal app code checker"? We could even add a reference to your app that disables it. Alternatively,  I can delete the sentence for now but think there will have to be some kind of documentation about this. Its a problem that there is so little user-facing info about these things. @jospoortvliet , what do you say?