[#785] docs: Add updating Go instructions

Signed-off-by: Pavel Karpy <carpawell@nspcc.ru>

docs/update-go-instruction.md

@@ -0,0 +1,35 @@
+# Updating Golang version
+
+## Update go.mod
+
+`go mod edit -go=X.Y`
+
+## Update CI
+
+Change Golang versions for unit test in CI.
+There is `go` section in `.github/workflows/go.yaml` file:
+```yaml
+jobs:
+  test:
+    runs-on: ubuntu-20.04
+    strategy:
+      matrix:
+        go: [ 'X.Y.x', 'X.Y.x' ]
+```
+
+That section should contain two latest Golang minor versions
+that are currently supported by Golang authors.
+
+## Update docker images
+
+Update all docker files that contain `golang` image in `./docker`
+directory.
+
+## Update Makefile
+
+Update `GO_VERSION` variable in `./Makefile`.
+
+## Apply language changes
+
+Open PR that fixes/updates repository's code according to 
+language improvements.