Update the versioning document to mention co0

Signed-off-by: Andrej Shadura <andrew.shadura@collabora.co.uk>

content/architecture/versioning.md

@@ -51,11 +51,16 @@ how would that be named so that version-revision will not conflict with other
 changes? What about if we want to backport the version in development, to build
 against stable branch? What about us, as a downstream of the Debian package,
 how should we version the package if we want to apply some changes on top of
-Debian package?  The convention, when modifiying the package for security
+Debian package?  The convention, when modifying the package for security
 updates, backports and downstream modification, is to append to the end of the
-existing Debian version number. As a result of this policy, many packages in
+existing Debian version number. As a result of this policy, packages in
 Apertis bear the addition `coX`, where `X` is a incremented number, which
 shows the number of modifications made to the package by Collabora for Apertis.
+The `co0` suffix means that the only difference between the upstream package
+from Debian and the package in Apertis is the metadata under `debian/apertis/`
+and the changelog entry itself. This is to highlight the fact that this metadata
+ends up in the generated source package, so this source package carries a
+small delta against the corresponding Debian package.
 
 Additionally, there are a number of symbols that are used to separate these
 portions of the revision.  The symbol `~` is used to infer "less", and `+` for