Update CONTRIBUTING.md w/ stipulation that go 1.13 is not yet supported

.github/CONTRIBUTING.md

@@ -60,7 +60,7 @@ issue. Stale issues will be closed.
 ## Building Consul
 
 If you wish to work on Consul itself, you'll first need [Go](https://golang.org)
-installed (version 1.12+ is _required_). Make sure you have Go properly installed,
+installed (version 1.12 is _required_, [go 1.13 is not yet supported](https://github.com/hashicorp/consul/issues/6879)). Make sure you have Go properly installed,
 including setting up your [GOPATH](https://golang.org/doc/code.html#GOPATH).
 
 Next, clone this repository into `$GOPATH/src/github.com/hashicorp/consul` and