Update and clarify example

[mdpiper]

This PR updates the long example where we show how to babelize the bmi-example-c library.

It:

• adds a figure depicting the directory structure when the example is finished,
• uses relative paths to move between steps in the example, and
• shows how to make a config file for the example at the command line.

I also touched up the text in a few spots.

This fixes #51.

[lheagy]

This looks great @mdpiper! These changes address all of my comments in #51

[mdpiper]

Thanks @lheagy! I'll merge this PR and close the referring issue.