Added Advertisement print columns

[fraborg]

Hi guys!! 🎆 🎆

A very small PR after a lot of time 😄

Description

Kubebuilder allows to specify additional print columns when you perform kubectl get <crd>( link )
In this PR some additional columns have been added to the Advertisement to easily find its status (Accepted/Refused), expiration date and if the VirtualKubelet has been created. Different/other fields can be easily added.
A dead link to Contributing page has been updated too.

How Has This Been Tested?

Tested with our beloved KinD

[adamjensenbot]

Hi @fraborg. Thanks for your PR!

I am @adamjensenbot.
You can interact with me issuing a slash command in the first line of a comment.
Currently, I understand the following commands:

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Make sure this PR appears in the liqo changelog, adding one of the following labels:

• kind/breaking: 💥 Breaking Change
• kind/feature: 🚀 New Feature
• kind/bug: 🐛 Bug Fix
• kind/cleanup: 🧹 Code Refactoring
• kind/docs: 📝 Documentation

[mlavacca]

HI @fraborg! Thanks for this PR 😀 your contribution is always warmly welcome!

[palexster]

/merge