Add tfbackend extension to tfvars syntax (#102)

* Add tfbackend to tfvars syntax

* Update link for tfvars documentation

Author: michaelblyons

Terraform-vars.sublime-syntax

@@ -22,6 +22,9 @@ extends: Terraform.sublime-syntax
 # File Extensions:
 #
 # - ".tfvars": variables definitions file
-#   https://www.terraform.io/docs/configuration/variables.html#variable-definitions-tfvars-files
+#   https://developer.hashicorp.com/terraform/language/values/variables#variable-definition-files
+# - ".tfbackend": backend configuration file
+#   https://developer.hashicorp.com/terraform/language/backend#file
 file_extensions:
   - tfvars
+  - tfbackend