Added framing/motivation for the paper

[DamienIrving]

This is my attempt at addressing #47.

I haven't addressed the suggestion regarding "coherency of modules made by contributors that likely span a range of experience in a given discipline" because I think rule 3 ensures coherence.

I also haven't addressed "the reluctance of academic administrators to have their students taught with course materials developed by third parties", because the paper is short, sharp and concise and generally avoids getting sidetracked on peripheral issues such as this.

Happy to hear suggestions to the contrary if people think these particular concerns should be addressed.

[raynamharris]

You and Ian had edited the same sentence, but different parts. So the conflict resolution was easy.