command: v0.12.0-beta1 supplement to the "no compatible providers" me…

…ssage

Since v0.12.0-beta1 is being released before all providers have stable
releases compatible with it, we will include a link to the announcement
blog post which includes information on how to install the development
snapshots of most providers that are not yet ready for auto-install.

command/init.go

@@ -918,8 +918,11 @@ move to a newer major release of this provider. Alternatively, if you are
 attempting to upgrade the provider to a new major version you may need to
 also upgrade Terraform to support the new version.
 
-Consult the documentation for this provider for more information on
-compatibility between provider versions and Terraform versions.
+Terraform v0.12.0-beta1 was released before all providers were updated for
+compatibility. For more information on provider availability for beta1,
+including workarounds for testing with not-yet-supported providers, please see
+the release announcement:
+    https://www.hashicorp.com/blog/announcing-terraform-0-1-2-beta1
 `
 
 const errProviderInstallError = `