Added info on the build system to contributing guide

I recently wrote a blog post on contributing to the Rust compiler which
gained some interest. It was mentioned in a comment on Reddit that it
would be useful to integrate some of the information from that post to
the official contributing guide.

This is the start of my efforts to integrate what I wrote with the
official guide.

This commit adds information on the build system. It is not a complete
guide on the build system, but it should be enough to provide a good
starting place for those wishing to contribute.

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