README: Erkläre, wie man den Kalender lokal sieht.

`hugo serve` rendert die Tabelle „Nächste Veranstaltungen" auf der
Startseite nicht — die wird erst durch `./build.sh` per sed in
`public/index.html` eingefügt. Die README beschreibt jetzt den
Build-und-Servieren-Workflow inklusive Python-Abhängigkeiten und
benötigtem Locale.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>
Signed-off-by: Hauke Mehrtens <hauke@hauke-m.de>

README.md

@@ -27,10 +27,24 @@ This is the website of the CCCB.
 
 5. Point your browser to: <http://localhost:1313/>
 
-To ready your site for upload, run `./build.sh`, which also generates `all.ics`
+Every change you make on the project will be reflected in your browser as long
-and adds the calendar table to `index.html`.
+as `hugo serve` is running.
-Every change you make on the project will be reflected in your browser
+
-as long as `hugo serve` is running.
+The *"Nächste Veranstaltungen"* table on the home page is generated by
+post-processing in `./build.sh`, not by Hugo, so it is **not** visible under
+`hugo serve`. To preview the fully-built site (including the home-page
+calendar), or to ready the site for upload, run:
+
+```shell
+./build.sh
+python3 -m http.server -d public 1313
+```
+
+`build.sh` generates `all.ics` and replaces the `CALENDAR` placeholder in
+`index.html` with the upcoming-events table. It depends on Python with the
+`icalendar`, `python-dateutil`, and `pytz` packages, plus a `de_DE.UTF-8`
+locale (used to format weekday names). Inside `nix develop` these are
+provided automatically.
 
 To build with *nix*: `nix build '.?submodules=1#production-content'`