readme: specify module path in go mod init command

Fixes error:

	go: cannot determine module path for source directory
	/home/redfish/foo/ncdns (outside GOPATH, module path must be
	specified)

Fixes #141

README.md

@@ -112,7 +112,7 @@ Option B: Using Go build commands with Go modules (works on any platform with Ba
 2. Run the following in the ncdns directory to set up Go modules:
    
    ~~~
-   go mod init
+   go mod init github.com/namecoin/ncdns
    go mod edit -replace github.com/coreos/go-systemd=github.com/coreos/go-systemd/v22@latest -replace github.com/namecoin/x509-compressed=../x509-compressed
    go mod tidy
    ~~~