README.md: add upgrade notice

Signed-off-by: Enrico Weigelt, metux IT consult <info@metux.net>

README.md

@@ -15,6 +15,27 @@ groups, state actors, etc. It's explicitly free of any "DEI" or similar
 discriminatory policies. Anybody who's treating others nicely is welcomed.
 
 
+Upgrade notice
+--------------
+
+* Module ABIs have changed - drivers MUST be recompiled against this Xserver
+  verison, otherwise the Xserver can crash or not even start up correctly.
+
+* If your console is locked up (no input possible, not even VT switch), then
+  most likely the input driver couldn't be loaded due version mismatch.
+  When unsure, Better be prepared to ssh into your machine from another one
+  or set a timer that's calling `chvt 1` after certain time, so you'll don't
+  need a cold reboot.
+
+* Proprietary NVidia drivers might break: they still haven't managed to do
+  do even simple cleanups to catch up with Xorg master for about a year.
+  All attempts to get into direct contact have failed. We're trying to work
+  around this, but cannot give any guarantees.
+
+* Most xorg drivers should run as-is (need recompile!), with some exceptions.
+  See .gitlab-ci.yml for the versions/branches built along w/ Xlibre.
+
+
 Contact
 -------