docs: add link in README to new adding-new-resource guide

[displague]

Description of your changes

Follow-up to #405

Adds a link to the new guide to the README where all other docs are currently listed.

Fixes #

I have:

• [x] Read and followed Upjet's contribution process.
• [ ] Run make reviewable to ensure this PR is ready for review.
• [ ] Added backport release-x.y labels to auto-backport this PR if necessary.

How has this code been tested

[jeanduplessis]

Thanks for this update @displague