Add a note about writing new mappers

CONTRIBUTING.md

@@ -58,3 +58,15 @@ When running docker image via `make run-docker` with `DEBUG` make variable set
 to `true` volume mapping will be added (in read-only mode), so that unsee
 instance running inside the docker can read asset files from the sources
 directory.
+
+## Adding support for new Alertmanager release
+
+To support a new release that breaks API following changes needs to be done:
+
+* Verify that `GetVersion()` function can still correctly read remote
+  Alertmanager version via `/api/v1/status` endpoint, adapt it if needed.
+* Create a new mapper package implementing unmarshaling of alerts and/or
+  silences (depending if both need a new code) under mapper/vXY (X major
+  Alertmanager version, Y minor version).
+* Register new mapper in the `init()` function in the
+  `alertmanager/alertmanager.go` file.