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.
**that run in the server instance**: I'd get rid of that
**A repository of apps**: An app repository/One app repository
**Apps**: lowercase, multiple occurences
**then show this app in the list**: then show **the** app in the list of installable apps ~~in a column at the left of the ownCloud webinterface~~