Add something about comps.xml

docs/koji/bootstrapping.md

@@ -173,6 +173,10 @@ Added external repo cs8-testing-appstream to tag cern8s-build (priority 35)
 Added external repo cs8-testing-powertools to tag cern8s-build (priority 45)
 ```
 
+The `cern` tag is special because it includes a [comps.xml](https://gitlab.cern.ch/linuxsupport/comps.xml)
+file, which sets up certain groups of packages. You'll have to add an xml file
+in that repo, creating the named groups. Without those groups, repo validations
+done in the snapshotting scripts will fail.
 
 ## Create base for user tags
 
@@ -367,6 +371,16 @@ These packages will be needed for openafs:
 Once you get to the Puppet part, you'll also need these RPMs:
    * [`cern-krb5-conf`](https://gitlab.cern.ch/linuxsupport/rpms/cern-krb5-conf)
 
+The `cern` tag's [`comps.xml`](https://gitlab.cern.ch/linuxsupport/comps.xml)
+file will probably list other RPMs you need to build, such as:
+   * [`cern-anaconda-addon`](https://gitlab.cern.ch/linuxsupport/rpms/cern-anaconda-addon)
+   * [`hepix`](https://gitlab.cern.ch/linuxsupport/rpms/hepix)
+   * [`cern-get-sso-cookie`](https://gitlab.cern.ch/linuxsupport/rpms/cern-get-sso-cookie)
+   * [`yum-autoupdate`](https://gitlab.cern.ch/linuxsupport/rpms/yum-autoupdate)
+   * [`sicgsfilter`](https://gitlab.cern.ch/linuxsupport/rpms/sicgsfilter)
+   * [`lpadmincern`](https://gitlab.cern.ch/linuxsupport/rpms/lpadmincern)
+   * [`cern-config-users`](https://gitlab.cern.ch/linuxsupport/rpms/cern-config-users)
+
 ## Create tag for openafs
 
 We need a tag for the openafs kernel modules, we can create it using the