Document tzdata as a dependency

[bdarnell]

Certain features of CockroachDB require the tzdata package (sometimes called zoneinfo) to be installed (specifically, the ability to use place names as time zone identifiers). We had thought this was installed by default in all major linux distributions but in cockroachdb/cockroach#32415 (comment) an ubuntu user reported that it was missing. We should document this dependency (and recommend that it be kept up to date, since the time zone/DST rules change several times a year).

[jseldess]

@lhirata, I can take this one.