[REG-1635] Add SDK link to homepage

[vontell]

Adds the SDK url to the homepage for power users who want to jump right in. Assumes they know how to add git packages to Unity through the package manager. 

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Every reviewer and the owner of the PR should consider these points in their request (feel free to copy this checklist so you can fill it out yourself in the overall PR comment)

• The code is extensible and backward compatible
• New public interfaces are extensible and open to backward compatibility in the future
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[rcornwall-reg]

[Non Blocking]
Linking directly to the package instead of the root project is a good idea, but I feel like we should have another README inside the package folder as well. It's always a bit jarring to me when I hit a repo and there's no README

[nAmKcAz]

Approving, but the number of users that would do this without first reading what the package does seems like it would be small or near zero.

[vontell]

For reviewers: @rcornwall-reg the repo does have a readme, although it's in the root folder, not in the package subfolder.

@RG-nAmKcAz definitely see what you are saying - I have found that I do use this option in other packages though when I've used them before - when I know what I want, I've used it before, and I don't want to dig through the docs for the package link.

[rcornwall-reg]

@vontell I was specifically talking about adding a second README to the subfolder you're directly linking to, at least with install instructions