initial commit of instructions for building from source, addresses #22

This is my first cut at a set of instructions for building machinekit from source.

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The `next_section: community` implies this will be inserted into the "Documentation/Getting Started" section, second item?  How about in the "Documentation/Development" section, first item, so `next_section: contributing`?

Wherever this ends up, it also needs entry into `_data/docs.yml` for the link to show up on the side bar.