README: set habitat channel to unstable

[Artoria2e5]

The hab I just downloaded defaults to the stable channel, but there are no stable releases for this package. Use the only channel that exists.

[jgavris]

@themightychris any thoughts?

[jgavris]

@Artoria2e5 could you squash your commits if this gets merged?

[themightychris]

no don't do this, the origin owner can promote any build to stable

[themightychris]

I maintain a stable build in my origin, maybe it makes more sense just to use mine in the guide? https://bldr.habitat.sh/#/pkgs/jarvus/rs-git-fsmonitor/latest

Or, you could add me as an admin to your origin and I could maintain it there instead

[jgavris]

@themightychris I made you a maintainer if you'd like to merge this.