Update build README

builder/README.md

@@ -6,9 +6,10 @@ Docker CE
 # builder/build-package <os> <os_codename> <build_tag>
 # os_codename is what "lsb_release -c" outputs, e.g. buster, focal.
 #
-# build_tag allows building multiple versions of deb package targeting a single
-# distro release (e.g. Ubuntu Bionic). If build_tag is given, the package name
-# will include build_tag as part of Debian revision. For example:
+# build_tag allows building multiple versions of deb package (rpm not supported)
+# targeting a single distro release (e.g. Ubuntu Bionic). If build_tag is given,
+# the package name will include build_tag as part of Debian revision. For
+# example:
 # * with build_tag: kasmvncserver_0.9.1~beta-1+libjpeg-turbo-latest_amd64.deb
 # * without build_tag: kasmvncserver_0.9.1~beta-1_amd64.deb
 # You need to have .build and .deb.build for the build_tag.