Updating readme to indicate prerelease

[swernli]

@BenjaminArmstrong
This is in response to #22

[msftclas]

Hi @swernli, I'm your friendly neighborhood Microsoft Pull Request Bot (You can call me MSBOT). Thanks for your contribution!

It looks like you're a Microsoft contributor (Stefan Wernli). If you're full-time, we DON'T require a Contribution License Agreement. If you are a vendor, please DO sign the electronic Contribution License Agreement. It will take 2 minutes and there's no faxing! https://cla.microsoft.com.

TTYL, MSBOT;

[swernli]

@BenjaminArmstrong Let me know if this looks good, and I'll merge it.
@jstarks @jterry75 FYI.

[BenjaminArmstrong]

Looks good - thanks!