docs(readme): --why, on the page the 17 consuming handles actually read

The quickstart is where a session learns the CLI, so the announcement
verb has to be in the first code block rather than in a section further
down that nobody scrolls to. States the trade plainly: optional, nothing
breaks without it, and a booth that cannot say what it is has no way to
ask for attention except by posting its URL somewhere else.

README.md

@@ -22,9 +22,9 @@ filesystem *is* the state.
 A booth is **just a folder** under the data dir. Three ways, cheapest first:
 
 ```bash
-# 1. On nh3-dev — the helper (services/booth/scripts/booth):
+# 1. On nh3-dev — the helper (scripts/booth):
-booth add my-run  out/a.png out/b.png       # creates booth + copies, prints URL
+booth add my-run out/a.png out/b.png --why "pick the denoiser, v3 on the left"
-booth new  my-run                            # empty booth, then cp/mv into ~/booth-data/my-run/
+booth new  my-run --why "..."                # empty booth, then cp/mv into ~/booth-data/my-run/
 booth url  my-run                            # just print the URL
 booth ls                                     # list booths
 booth rm   my-run                            # wipe now (TTL would anyway)
@@ -39,6 +39,25 @@ rsync -a ./out/  nh3-dev:booth-data/my-run/
 
 Then hand the operator `http://10.100.10.50:8090/b/my-run/`.
 
+### Say what it is — `--why`
+
+**`--why` is one line telling the operator what he is looking at and why.** It
+lands on the index card and on the booth page next to your handle (taken from
+`$ALTHING_HANDLE`), stored as `.booth.json` in the booth.
+
+It is optional and nothing breaks without it — a booth with no announcement
+renders as `unannounced`, which is also what every booth created by `rsync` or
+a bare `mkdir` looks like. But a booth that cannot say what it is has no way to
+ask for attention except by posting its URL somewhere else, and that is exactly
+how the link board ended up 69% dead rows. **The booth is the place to say it.**
+
+```bash
+booth add r18-ab out/*.png --why "which denoiser — v3 left, v4 right" --title "R18 A/B"
+```
+
+A second `new` or `add` on the same booth updates the why and keeps the
+original creation stamp: the booth appeared once.
+
 ## Checking that controls can actually be clicked
 
 ```bash