Clarify supported Terraform versions (#677)

Author: nicholasklick

README.md

@@ -14,7 +14,7 @@ Terraform Provider for Gitlab
 Requirements
 ------------
 
--	[Terraform](https://www.terraform.io/downloads.html) 0.12.x
+-	[Terraform](https://www.terraform.io/downloads.html) >= 0.12.x
 -	[Go](https://golang.org/doc/install) >= 1.14 (to build the provider plugin)
 
 ## Developing The Provider