add Developer's Mental Health talk proposal

as most people asked for that topic - here is my proposal.

Was asked from others about time for disucssion about that topic, as it is a sensible one. There could be the need to talk about things after my talk (but perhaps there is no need, I dont know). We should have enough free time to enable people talking about that topic.

not sure, if we should use an extra slot for that - just wanted to notice it here.