Update repository links in README.md

[spier]

This PR contributes to #130 but does not implement it entirely.

Proposed Changes

• Update repository links in README.md
  from: meltwater/drone-cache
  to: drone-plugins/drone-meltwater-cache

Description

Checklist

• Read the CONTRIBUTING document.
• Read the CODE OF CONDUCT document.
• Add tests to cover changes.
• Ensure your code follows the code style of this project.
• Ensure CI and all other PR checks are green OR
  • Code compiles correctly.
  • Created tests which fail without the change (if possible).
  • All new and existing tests passed.
• Add your changes to Unreleased section of CHANGELOG.
• Improve and update the README (if necessary).
• Ensure documentation is up-to-date. The same file will be updated in plugin index when your PR is accepted, so it will be available for end-users at http://plugins.drone.io.

[spier]

The label "storage-backend" does not exist anymore. (this repo was forked and the issues are not carried over from the original repo)

[spier]

Many of the icons at the very top of the README also point to the previous repository where this was hosted, however I don't know how to fix those.