Add AKS installation walkthrough

[jeremyrickard]

Fixes #208

Proposed Changes

• This PR adds a walkthrough providing instructions on how to install knative
  on Azure Kubernetes Service.
• It also adds a little more wording to the opening
  of README.md.

[googlebot]

Thanks for your pull request. It looks like this may be your first contribution to a Google open source project (if not, look below for help). Before we can look at your pull request, you'll need to sign a Contributor License Agreement (CLA).

📝 Please visit https://cla.developers.google.com/ to sign.

Once you've signed (or fixed any issues), please reply here (e.g. I signed it!) and we'll verify it.

---

What to do if you already signed the CLA

Individual signers

• It's possible we don't have your GitHub username or you're using a different email address on your commit. Check your existing CLA data and verify that your email is set on your git commits.

Corporate signers

• Your company has a Point of Contact who decides which employees are authorized to participate. Ask your POC to be added to the group of authorized contributors. If you don't know who your Point of Contact is, direct the Google project maintainer to go/cla#troubleshoot (Public version).
• The email used to register you as an authorized contributor must be the email used for the Git commit. Check your existing CLA data and verify that your email is set on your git commits.
• The email used to register you as an authorized contributor must also be attached to your GitHub account.

[jeremyrickard]

Oooops, I'l get myself added to the Microsoft CLA.

[jeremyrickard]

I signed it!

[googlebot]

CLAs look good, thanks!

[rgregg]

Suggest wrapping this command line, since you have to scroll to read it in the markdown view.

[jeremyrickard]

I'll make that change and push it up. Thanks! 👍

[rgregg]

/lgtm

[rgregg]

/lgtm
/approve

[google-prow-robot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: jeremyrickard, rgregg

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

The pull request process is described here

Needs approval from an approver in each of these files:

• OWNERS [rgregg]

Approvers can indicate their approval by writing /approve in a comment
Approvers can cancel approval by writing /approve cancel in a comment