README: add v2 and v3 install instructions

In preparation for joining the master.v3 branch back into master.

README.md

@@ -16,7 +16,14 @@ For high-level operations like performing shell expansions on strings, see the
 
 ### shfmt
 
-	go get -u mvdan.cc/sh/v3/cmd/shfmt
+Go 1.11 and later can download the latest v2 stable release:
+
+	cd $(mktemp -d); go mod init tmp; go get mvdan.cc/sh/cmd/shfmt
+
+The latest v3 pre-release can be downloaded in a similar manner, using the `/v3`
+module:
+
+	cd $(mktemp -d); go mod init tmp; go get mvdan.cc/sh/v3/cmd/shfmt
 
 `shfmt` formats shell programs. It can use tabs or any number of spaces to
 indent. See [canonical.sh](syntax/canonical.sh) for a quick look at its default