Add info.rkt for the parsack collection.

For scribblings. See issue #42. Although I think adding this is
necessary, I'm not sure it's sufficient: Although I do now see
`raco setup parsack` building parsack.scrbl, and an XREPL
`,doc parse-result` works, I don't see it using Search in the web
browser. So either something else remains to be done, or, it was
sufficient and I just have a stale cache.