Add go generate directive

At this stage this is just a comment for users. The directive will have
effect at go tip when go CL125580044 is submitted. When that happens, at
tip parser code generation can be acheived in the default case with:

    go generate

in the relevant package directory.

quad/cquads/cquads.go

@@ -12,6 +12,8 @@
 // See the License for the specific language governing permissions and
 // limitations under the License.
 
+//go:generate ragel -Z -G2 parse.rl
+
 // Package cquads implements parsing N-Quads like line-based syntax
 // for RDF datasets.
 //