Add dependency update documentation

Add vendorind and dependency update documentation with
vendoring process of ARO-RP.

Signed-off-by: Petr Kotas <pkotas@redhat.com>

docs/updating-dependencies.md

@@ -0,0 +1,62 @@
+# Updating dependencies
+
+To update golang dependencies one has to choose between two options:
+
+1) update the whole dependency tree
+2) update or add what is currently required
+
+
+## Update the whole dependency tree
+
+Done when the need to catch upstream dependencies arises, it can be done by
+calling
+
+```bash
+make vendor
+```
+
+in root folder, which calls `hack/update-go-module-dependencies.sh`.
+
+The reason for calling script instead of directly calling:
+
+```bash
+go get -u ./...
+go mod tidy
+go mod vendor
+```
+
+is that packages modified in this script do not fully support modules and
+semantic versioning via tags. Therefore the proper version is parsed from the version
+branch and fixed using replace directive. Otherwise it will upgrade every time
+the command is started.
+
+When upgrading to a never version of OpenShift, this script have to be updated to
+reflect the proper release.
+
+
+## Update only required dependencies
+
+When adding new dependencies or updating old ones, based on the requirement of
+the PR, one can simply call
+
+```bash
+go get <module>@<release> OR
+go get -u <module>@<release>
+
+go mod tidy
+go mod vendor
+```
+
+---
+
+**NOTE** vendoring is required as ARO mirrors all dependencies locally for the CI reliability
+and reproducibility.
+
+**NOTE** that when running `go mod vendor` only modules that are used in the
+source code will be vendored in.
+
+**NOTE** when updating a package modified in `hack/update-go-module-dependencies.sh`
+changes have to be made there also. Otherwise next run of `make vendor` will
+overwrite new changes.
+
+More information on working with modules: https://blog.golang.org/using-go-modules