Added spectrum and camera specs

readme.md

@@ -42,6 +42,8 @@ milestones, each split into one issue per change:
 - `crates/resolved`: the library.
 - `crates/resolved-cli`: the `resolved` command, for rendering without writing
   a host program.
+- `docs/optics.md`: the optics and radiometry the renderer models, explained
+  for readers without an optics background.
 
 ## Development