Add cassandra to table in readme

[kragniz]

What this PR does / why we need it: add cassandra to table in readme

Which issue this PR fixes (optional, in fixes #<issue number>(, fixes #<issue_number>, ...) format, will close that issue when PR gets merged): fixes #

Special notes for your reviewer:

Release note:

NONE

[jetstack-ci-bot]

[APPROVALNOTIFIER] This PR is NOT APPROVED

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Assign the PR to them by writing /assign @munnerz in a comment when ready.

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[kragniz]

/test e2e

[jetstack-bot]

[APPROVALNOTIFIER] This PR is APPROVED

Approval requirements bypassed by manually added approval.

This pull-request has been approved by:

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

Needs approval from an approver in each of these OWNERS Files: - **[OWNERS](https://github.com/jetstack/navigator/blob/master/OWNERS)** You can indicate your approval by writing `/approve` in a comment You can cancel your approval by writing `/approve cancel` in a comment

[wallrj]

/test e2e

[wallrj]

/retest

[jetstack-ci-bot]

Automatic merge from submit-queue.