Adding "calls for contributors" to the newsletter

Recently mentioned in a different thread: there are a lot of opportunities for junior devs to contribute and join the community and a lot of need for more maintainers (even though the bus factor for julia projects is on average good).

So, whenever you feel such a need, please add a mention of it to the Julia Newsletter draft: Stefan's notes on the Julia world - Google Docs

Such mentions will be visible at the end of each month when the newsletter is posted.