Incorporate "tutorial-R-addlib" content

[aowen-uwmad]

The content of github.com/osgconnect/tutorial-R-addlib should be incorporated into this tutorial

• this tutorial is too short to justify having as a stand-alone page
• the first question a user will have is "what about my packages?"

Suggest the following outline:

1. Running R in the command line (tutorial-R)
2. Running R in an HTCondor job (tutorial-R)
3. Creating R container with desired packages (tutorial-R-addlib)
4. Running R in the command line with custom container (tutorial-R-addlib ?)
5. Running R in an HTCondor job using custom container (tutorial-R-addlib)

[aowen-uwmad]

Will be doing most of the work in the addlib-content branch.