Convert component guidelines to markdown (attempt #1)

This converts the component design guidelines to markdown so we can better evolve this document as a community.

It's an open question as to whether these guidelines should be under http://fsharp.org/specs or elsewhere (e.g. http://fsharp.github.io).  I'm leaving them here for now, with converted format.

The markdown will undoubtedly need some tweaking, e.g. because of the use of unicode characters etc.