Docs needed for PodSpec changes regarding readiness

[tengqm]

This is a...

• Feature Request
• Bug Report

Problem:
The new field readinessGates for Pods was introduced in kubernetes/kubernetes#64057. This is a very useful feature for testing the readiness of Pods thus deserves some documentation with examples.

Proposed Solution:
Document the new field.

Page to Update:

Release 1.11

[tengqm]

/assign