docs: fix Getting Started local-run example for the refused placeholder

The startup now rejects the config.example.yml placeholder signing_key,
so the documented `pixad --config config.example.yml` command aborts on
first run. Show copying the example to config.yml and setting a real
signing_key before running, matching the code.

Model: opus-4-8

README.md

@@ -15,8 +15,12 @@ git clone https://git.eeqj.de/sneak/pixa.git
 cd pixa
 make build
 
-# run with a config file
+# run with a config file: copy the example and set a real signing key
-./bin/pixad --config config.example.yml
+# (the example placeholder is refused at startup), e.g. with
+# openssl rand -base64 32
+cp config.example.yml config.yml
+$EDITOR config.yml   # replace the signing_key placeholder
+./bin/pixad --config config.yml
 
 # or build and run via Docker
 make docker