feature: highlight a repo participating in hacktoberfest

[andrewhsu]

As of this writing, Hacktoberfest this year is 20 days away...

I propose a feature for this occasion: if a repo has a topic hacktoberfest, and the current date is during the month of October, have an additional message returned to the user to highlight hacktoberfest participation, e.g.

bash$ gh contribute
A good issue to work on in cli/cli might be...

Issue #3466: linter rule: ID fields are int64

You can read the issue with `gh issue view -Rcli/cli 3466`
Or express interest in working on it: `gh issue comment -Rcli/cli 3466 -b"Hi! I'm interested in working on this"`

View this issue on the web: https://github.com/cli/cli/issues/3466

🎃 This repo has opted-in for Hacktoberfest 2021! For more info on participating in this open source festival,
visit: https://hacktoberfest.digitalocean.com

Reference: https://hacktoberfest.digitalocean.com/resources/participation