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.
@Raydiation Unfortunately there are many apps which do not have a github repository. Also, being pointed to github, having to create yet another account and being confronted with communication that gets quite technical and specific is suboptimal for people who just want to use ownCloud. One of the reasons I found it necessary to write this addition to the documentation is that i found myself tossed back and forth between github and the apps.owncloud.com comment section.Sometimes githup repositiry members are less than welcoming of n00bs asking "simple" questions, some others say they don't want to check the app.owncloud.com comments...I think for most users using the comment section would be a logical step. I know that the finctionality of teh comment section and teh app store is limited, but wouldnt that rather be something for owncloud to improve, rather than requiring users to jumt between different issue trackers and comment systems? My szggestions would be questions>comment section, bugs>github.