Go Get from README Fails

[tri-adam]

The README.md contains instructions on how to get the module:

sif/README.md

Lines 17 to 21 in a7d8315

	To get the sif package to use directly from your programs:
	```sh
	go get github.com/sylabs/sif/v2
	```

As of v2.0.0-beta.1, that command returns an error:

$ go get github.com/sylabs/sif/v2
github.com/sylabs/sif/v2: no Go source files
package github.com/sylabs/sif/v2: build constraints exclude all Go files in /home/adam/src/sif

It looks like this will resolve itself in a future version of Go (golang/go#29268), but in the meantime we should fix our docs.