Add more SASL details to README

It took me a while to setup SASL on my machine, mostly because of
non-installed SASL modules. I expanded README a little bit to
note this.

Signed-off-by: Christophe Fergeau <cfergeau@redhat.com>
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Name Stage Failure
failed
centos-8-git-build Builds
	/builds/libvirt/libvirt-go/api_test.go:504 +0x551
testing.tRunner(0xc0000ae120, 0x632f48)
/usr/lib/golang/src/testing/testing.go:1039 +0xdc
created by testing.(*T).Run
/usr/lib/golang/src/testing/testing.go:1090 +0x372
exit status 2
FAIL libvirt.org/libvirt-go 0.083s
Cleaning up file based variables
ERROR: Job failed: exit code 1