README: fix compile instructions for Go 1.13+

"go get -d" does not download to GOPATH/src anymore:
golang/go#31529

Use explicit "git clone" to the current directory as suggested
in golang/go#31529 (comment) .

Fixes #553

README.md

@@ -96,10 +96,10 @@ as well as in the go-fuse library.
 Compile
 -------
 
-With [go 1.11 or higher](.travis.yml#L12):
+With go 1.11 or higher:
 
-	$ go get -d github.com/rfjakob/gocryptfs
-	$ cd $(go env GOPATH)/src/github.com/rfjakob/gocryptfs
+	$ git clone https://github.com/rfjakob/gocryptfs.git
+	$ cd gocryptfs
 	$ ./build.bash
 
 build.bash needs the OpenSSL headers installed (Debian: `apt install libssl-dev`,