docs: add feature gates to our readme

[tobiasgiese]

This PR adds a feature gates section to our README.
Depends on #733

[github-actions]

Thanks for your PR,
To run vendors CIs, Maintainers can use one of:

• /test-all: To run all tests for all vendors.
• /test-e2e-all: To run all E2E tests for all vendors.
• /test-e2e-nvidia-all: To run all E2E tests for NVIDIA vendor.

To skip the vendors CIs, Maintainers can use one of:

• /skip-all: To skip all tests for all vendors.
• /skip-e2e-all: To skip all E2E tests for all vendors.
• /skip-e2e-nvidia-all: To skip all E2E tests for NVIDIA vendor.
  Best regards.

[coveralls]

Pull Request Test Coverage Report for Build 10350466058

Details

• 0 of 0 changed or added relevant lines in 0 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage increased (+0.007%) to 45.161%

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Totals
Change from base Build 10318797846:	0.007%
Covered Lines:	6528
Relevant Lines:	14455

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💛 - Coveralls

[adrianchiris]

minor nit, overall LGTM

[tobiasgiese]

We should wait until #733 is merged IMO