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.
There are *NO* rejections at all because malicious code, there are only rejections because the code checker assumes usage of the private API. (which can be as well false positives)

That sentence needs to be rephrased.