README updates

Hodgepodge of small and major changes. Changed terminology to what I feel is much more explicit and easier to understand, so please review carefully.

When reading through the document carefully, I really struggled with the distinction between the workers and the fittings processes. In hindsight it's obvious, but it wasn't on first (or second) reading through the first few sections.

Also, in a couple of places the README file failed to include "process" when talking about fittings processes, so there was some unnecessary ambiguity between the concept, the metadata, and the processes themselves.

Rather than just tweak those references, I decided to rename "fittings process" to "coordinator". I feel this makes the role much more explicit, plus we use "process" so often (verb and noun) I think it helps with the flow. Note that I also shortened "worker process" to "worker" in the very few places where the former occurred.

I also introduced "sink" earlier, because it seemed like a natural fit while introducing other key actors.

Finally, there were various minor glitches to fix, such as a broken link and section titles with underscores that needed to be escaped for HTML generation to work properly.