fix(docs): remove outdated sections from CONTRIBUTING.md

CONTRIBUTING.md

@@ -8,8 +8,7 @@ file.
 ## Git tags and branches
 
 Every release tag name will follow `vX.Y.Z` naming scheme, example: `v0.1.0`.
-Every release tree has a dedicated branch named `release-X.Y`, example:
-`release-0.1`.
+All releases are tagged against the `master` branch.
 
 ## Commit messages
 
@@ -18,7 +17,7 @@ for all commits.
 
 ## Testing changes
 
-To run included tests and check code style with `golint` run:
+To run included tests and linters run:
 
     make test
 
@@ -59,15 +58,3 @@ 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 karma
 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
-  `internal/alertmanager/mapper.go` file.