Make corneliusweig a maintainer

[ahmetb]

Adding corneliusweig to owners. He has been consistently helping both with
krew and krew-index repositories in terms of:

• developing plugins himself
• taking a stab at krew machinery with large scale code refactors
• adding integration test suite to the project
• adding more validation and test cases
• increasing developer documentation

Some of his notable work:

• Save install receipt #195
• Validate license boilerplate in CI builds #183
• Validate manifest file extension and kind field #191
• Add contributor documentation and helper scripts #201
• Ensure index updates do not fail if the local and remote history diverge #202
• Add basic integration tests #203
• More integration tests #208

He is familiar with the codebase enough to officially review and approve code.

[k8s-ci-robot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: ahmetb

The full list of commands accepted by this bot can be found here.

The pull request process is described here

Needs approval from an approver in each of these files:

• OWNERS [ahmetb]

Approvers can indicate their approval by writing /approve in a comment
Approvers can cancel approval by writing /approve cancel in a comment

[codecov-io]

Codecov Report

Merging #214 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #214   +/-   ##
=======================================
  Coverage   53.77%   53.77%           
=======================================
  Files          13       13           
  Lines         716      716           
=======================================
  Hits          385      385           
  Misses        279      279           
  Partials       52       52

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update 95a9ad7...18ea1a6. Read the comment docs.

[corneliusweig]

Thanks @ahmetb for your trust in me. I hope to be a good steward for krew!

[ahmetb]

I went ahead and merged this as Juan seems to be away and Maciej isnt in OWNERS.