Update readme

[danmoseley]

Begin updating the root README with links to information about how to contribute.

The build requirements docs are already in this repo. The other docs are still links to corefx/coreclr docs that are still mostly applicable. This clearly isn't the end state, we can iterate here and over the next few days.

Is anyone planning to create equivalents of eg
https://github.com/dotnet/corefx/blob/master/Documentation/project-docs/developer-guide.md
https://github.com/dotnet/coreclr/blob/master/Documentation/project-docs/developer-guide.md

In general, is anyone driving coalescing all the markdown files? The near term goal should be to get sufficient docs up for the dev loop.

[danmoseley]

Ah, of course we brought all those docs over. I assume the near term goal is to just make sure those are updated, and longer term combine them if appropriate. I updated to local links.

So the first question is, is anyone planning to work on the developer-guide.md ?

[danmoseley]

Hmm, there is such a myriad of markdown files, and each repo did things a bit differently, I think it would be good to have a plan for a consistent set of key docs. As part of that it might be useful to look at how other repos did this.

[ViktorHofer]

Yeah I'll add building instructions (developer guide) tomorrow.