systemd: describe import of systemd code in README.md

src/libnm-systemd-core/README.md

@@ -7,10 +7,17 @@ as a static library with network helpers.
 We use systemd's DHCPv6 and LLDP library, by forking their code.
 
 We also still use their DHCPv4 library, but that is about to be replaced
-by nettools' n-dhcp4.
+by nettools' n-dhcp4 and not used unless you configure the undocumented
+`[main].dhcp=systemd` plugin.
 
 This approach of code-reuse is very cumbersome, and we should replace
-systemd code by a proper library (like nettools).
+systemd code by a proper library (like [nettools](https://github.com/nettools/)).
 
 We should not use systemd directly from our sources, beyond what
 we really need.
+
+
+Reimport Upstream Code
+----------------------
+
+Read [here](../libnm-systemd-shared/README.md#reimport-upstream-code).

src/libnm-systemd-shared/README.md

@@ -9,3 +9,34 @@ which contains systemd library with network tools (like DHCPv6).
 
 We should not use systemd directly from our sources, beyond what
 we really need to make get libnm-systemd-core working.
+
+
+Reimport Upstream Code
+----------------------
+
+We want to avoid deviations in our fork, and frequently re-import
+latest systemd version (every 4 to 8 weeks). The reason is that we
+frequently should check whether important fixes were done in upstream
+systemd, and the effort of doing that is half the work of just reimporting.
+Also, by reimporting frequently, we avoid deviating hugely (instead we only deviate
+largely).
+
+Of course this is cumbersome. We should replace systemd code with something else.
+
+To do a re-import, do:
+
+- checkout `systemd` branch.
+
+- Use the last commit message (`git commit --allow-empty -C origin/systemd`).
+  Then modify the commit message (`git commit --allow-empty --amend`). The
+  commit message contains a long script that is used to re-import the code.
+  Adjust the script in the commit message, and run it. Commit the changes on
+  `systemd` branch.
+
+- checkout `main` branch, and `git merge systemd`. Fix all issues, test,
+  repeat.
+
+- open merge request, check that all tests pass. In particular, enable build
+  on all test distributions.
+
+- push `main` and `systemd` branch.