File README.suse-maint of Package cargo-c

# How to update the cargo-c package

## Prerequisites:
You need the download_files and cargo_vendor obs services installed:
  zypper in obs-service-obs_scm obs-service-cargo_vendor

## Updating to a new version from upstream

Edit the _service file and update the revision parameter.

Download the new source file by running:
  osc service mr obs_scm

Run the cargo_vendor service with:
  osc service mr cargo_vendor

Update the changelog file with the upstream release notes.