Added Chicago Java User’s Group + include Mastodon link

[FDelporte]

A proposal to add Mastodon links, related to #10.
At this point we can conclude Mastodon is an important one to add.
I have no experience with the formatting so the if's in index.html may be wrong, but didn't manage to build on my PC so not sure...

[icougil]

Thank you Frank for your contribution 👍!
I think the links looks ok for the index.html. I'll prepare another PR to include the Mastodon link in the map 😉
I'm not 100% sure what we can do with the email field or where we should display it.... 🤔

[FDelporte]

thanks for merging and also adding to the map!
found out I didn't add target="_blank" to the links, so made a new merge request...

[icougil]

Thx Frank!
I already added the thing about opening a new window from the map.
I was not 100% sure if this is something we expect from the "simple list", but I saw you already created that PR ;-) (not an issue from my POV)