add package version number info

README.md

@@ -40,6 +40,30 @@ the CI/CD does: For each Debian release listed in the environment variable
 want to build for. Example: `sid bullseye`. Default: `sid buster
 bullseye`.
 
+## Package version numbers
+
+This CI/CD will build against each Debian distribution listed in
+`DEBIAN_RELEASES`. If the distribution is `sid` then `debian/changelog`
+is used as-is.
+
+However, if the distribution is _not_ `sid` then `debian/changelog`
+gets a new line with the message "Stanford backport to RELEASE." where
+"RELEASE" is the name of the Debian distribution. More importantly, the
+Debian version number is changed by appending a "backport" string.
+
+|Debian release|version string suffix|
+|buster        |`~sbp10+1`|
+|bullseye      |`~sbp11+1`|
+
+We do this because the Stanford repository does not allow two packages
+with the same version number even if the two packages are built against
+different distributions.
+
+Note 1: when there is a new Debian release the script
+[`generate-cicd-yaml.sh`](generate-cicd-yaml.sh) will need to be updated.
+
+Note 2: "sbp" stands for 'Stanford back-port".
+
 ## `dput` configuration
 
 Most of the complexity of the configuration centers around the `dput`