Update README.md
Updates the installation instructions regarding the presence of a `go.mod` file.
@@ -50,7 +50,8 @@ invoking the client code.
## Installation
-Assuming you have a working [Go](https://golang.org) setup:
+Assuming you have a working [Go](https://golang.org) setup, and are in a
+directory where a `go.mod` file is present:
```bash
export GO111MODULE=on
@@ -58,7 +59,14 @@ go get -u goa.design/goa/v3
go get -u goa.design/goa/v3/...
```
-or, when NOT using Go modules (this installs Goa v2, see below):
+If you don't have a `go.mod` file present, and only want to install the
+Goa command globally:
+
+```bash
+go get -u goa.design/goa/v3/cmd/goa
+```
+
+Alternatively, when NOT using Go modules (this installs Goa v2, see below):
```bash
go get -u goa.design/goa/...
