adds FORK_SYNC instructiones

FORK_SYNC.md

+In order to sync manually with the upstream project, you can follow these steps:
+- define an upstream remote pointing to the original project
+- fetch the upstream remote
+- merge the upstream remote with your local master branch
+
+Here is an example of how to do this:
+
+```bash
+$ git remote add upstream git@gitlab.com:gitlab-org/ci-cd/docker-machine.git
+$ git fetch upstream
+$ git checkout main
+$ git rebase upstream/main
+```
+