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.
yes, but there are issues with false positives, are there not? so saying "the app has been rejected by the internal app code checker" is more correct because not every rejection is due to malicious code? As I said I can remove this for now but want to flag this as "in need of better documentation". Because non-coder end users such as myself will come across this without there being enough info in the docs.