docs(readme): the worked example needs its catalog and sources

Run as written it stops at `no package source for readcon-core`, then at
`rgpot`: both are dependencies the robot does not carry, and the catalog and
the companion sources are what close them. With those two flags it emits the
application and both companion recipes.

Author: HaoZeke

README.md

@@ -214,6 +214,8 @@ eb-stack package plan \
   --toolchain-version 2026.1 \
   --package-config examples/packages/common.toml \
   --package-config examples/packages/eon.toml \
+  --package-catalog examples/package-catalog/eon.toml \
+  --package-sources examples/package-sources/eon-companions.toml \
   --easyconfigs /path/to/easybuild-easyconfigs/easybuild/easyconfigs \
   --easyconfigs fixtures/eon_foss_2026_1/easyconfigs \
   --stack-policy examples/stacks/eon-foss-2026.1.toml \