Add JetBrains support

[kelle]

JetBrains has been providing us with free licenses for many years. They have asked us this:

One small ask before I extend the subscriptions: would you be open to adding a “Supported by JetBrains” mention to your repo or on the website? It helps make the collaboration visible and supports the program. Here’s the snippet most participants use:

Supported by

Totally optional — we’ll be happy to extend your subscriptions either way. Looking forward to hearing from you!

I don't have a strong opinion either way.