docs: where this is going, and the site it belongs to

Adds a section placing this engine among the three, and pointing at
kirbyvtt.org. kirby-combat's link to an internal host is replaced with the
public site.

Author: pdbethke

README.md

@@ -152,6 +152,26 @@ character files. Both halves of that comparison have been wrong before and been
 corrected: the figure means "every value the corpus compares", and the corpus is
 described above so you can judge what that covers.
 
+## Where this is going
+
+This is one of three engines behind [Kirby](https://kirbyvtt.org), a virtual
+tabletop for the HERO System:
+
+- **[kirby-cost](https://github.com/pdbethke/kirby-cost)** — reads a HERO 6E
+  build and costs it, validated against Hero Designer
+- **[kirby-sheet](https://github.com/pdbethke/kirby-sheet)** — renders a
+  character to JSON, text, HTML, PDF, or back to `.hdc`
+- **[kirby-combat](https://github.com/pdbethke/kirby-combat)** — the combat
+  engine: attacks, movement, mental combat, vehicles, mass combat,
+  destructible terrain
+
+What's still to come is the table itself — terrain with its own PD and ED,
+elevation and concealment that move OCV and DCV, and line of sight worked out
+from where a character is actually standing. Kirby plays characters; it does
+not create them. Character creation stays in Hero Designer.
+
+Progress and notes at [kirbyvtt.org](https://kirbyvtt.org).
+
 ## Provenance
 
 **This is a port of licensed source code, purchased from Hero Games.**