Make alumni field non-optional

[dtolnay]

// Teams must contain an `alumni = […]` field (even if empty) so that there
// is an obvious place to move contributors within the same file when
// removing from `members`.
//
// Marker teams are exempt from this, as well as teams which comprise only
// members of other teams via `include-team-leads` or similar; they do not
// need `alumni = […]`. For these teams, the correct place to put alumni is
// in the same team they're being included from.

[rylev]

Seems fine to me. Can we add a note about this requirement to https://github.com/rust-lang/team/blob/master/docs/toml-schema.md?

[dtolnay]

Sure thing, good call. Done.