README: add requirement for build-tags when using non-current Ceph libraries
README: add requirement for build-tags when using non-current Ceph libraries

In order to selectively exclude/include certain .go files while building
and testing with the standard `go` tools, build-tags can be used.
Because go-ceph aims to support multiple Ceph versions, some functions
might not be available in the C libraries, or have been marked
deprecated in the header files. By selectively excluding/including some
of the .go files, warnings and errors can be prevented.

See-also: https://golang.org/pkg/go/build/#hdr-Build_Constraints
Signed-off-by: Niels de Vos <7bda0c0ee7f47171cc808e9e60292cbc5d8140dd@redhat.com>

ceph/go-ceph,ceph/go-ceph,ceph/go-ceph