State in the documetation that svc-cat depends on k8s v1.13+

[vpnachev]

This PR is a

• Feature Implementation
• Bug Fix
• Documentation

What this PR does / why we need it:
Improve documentation to make it clear that service catalog requires k8s cluster of version v1.13 or newer.

Which issue(s) this PR fixes

Fixes #2678

Please leave this checklist in the PR comment so that maintainers can ensure a good PR.

Merge Checklist:

• New feature
  • Tests
  • Documentation
• SVCat CLI flag
• Server Flag for config
  • Chart changes
  • removing a flag by marking deprecated and hiding to avoid
    breaking the chart release and existing clients who provide a
    flag that will get an error when they try to update

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[vpnachev]

/assign @MHBauer

[mszostok]

/ok-to-test

[mszostok]

/retest

[mszostok]

/lgtm

[MHBauer]

The other way to do this is to massage the language so that it mentions matching the k8s support process, so two releases-ish.

/approve

[k8s-ci-robot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: MHBauer, vpnachev

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[vpnachev]

The other way to do this is to massage the language so that it mentions matching the k8s support process, so two releases-ish.

@MHBauer
Actually, it should be a compatibility matrix like the client-go one, because svc-cat mostly increases the patch numbers, not the minor. For example, it is hard to define which are the supported k8s versions for svc-cat v0.1.x which was initially released 2 years ago.