Readme references outdated website with community link.

[Angular-Angel]

The community link at the bottom of the readme links to https://sandstorm.io/community instead of https://sandstorm.org/community.

[ocdtrekkie]

This is probably safe to change now that sandstorm.io itself also points to sandstorm.org, especially to refer to the community, but generally speaking we have been planning to update these once we are able to launch a community-released build.