README.md should mention cron event source

[krancour]

The README.md currently lists other official brigade projects that are compatible with Brigade v2.

idk if we'll eventually get away from doing that, but for now, the cron event source is conspicuously absent from the page.

Let's add it: https://github.com/brigadecore/brigade-cron-event-source

[hamees-sayed]

Do we need to add cron-event-source as a bullet point under a new "Event Source" Heading?

[krancour]

Ah, yes. Gateways are a type if event source, but not all event sources are gateways. A new heading after gateways would make sense. Let's have it say "Other Event Sources." The noisy neighbor could also be moved into that list.