README: add note on version bumps

Since these packages are a bit "special", the way they should be
bumped is now specified in the README. I hope the explanation is clear
enough.

Signed-off-by: Wolfgang Bumiller <w.bumiller@proxmox.com>

README.md

@@ -11,3 +11,31 @@ NOTE: You may need to adapt the perl version number in this path:
 ```
 
 Then just restart pvedaemon/pveproxy after running `make pve`.
+
+# Version Bumps
+
+## TL;DR
+
+- Common *code* changes -> bump the *products*, *not* the common package.
+- Common *package list* changes -> bump common AND products
+  (That's the `$(PERLMOD_PACKAGES)` list in `common/pkg/Makefile`)
+- On breaking changes to the common code, both PVE and PMG should be bumped along with the common
+  package which should declare its `Depends`/`Breaks` accordingly.
+
+## The `common` package.
+
+This package only provides the "list" of packages the common code contains, but does not by itself
+provide a *library*. In other words, this only provides a bunch of `.pm` files which cause whichever
+product-specific library is available to be loaded.
+
+The rust code in the common directory is compiled as part of the product libraries. Those libraries
+actually provide both the product specific as well as the common perl "packages" via the library.
+
+## The product packages.
+
+These are the actual libraries. Their source code both "include" the `common` rust code by way of a
+symlink (but when a source package is created, that symlink is replaced by a *copy* of the common
+dir), so building them locally with `cargo build` should use the `common` code from git.
+
+These provide the actual *functionality* the `common` package declares to exist (via its `.pm`
+files), in addition to the product specific parts.