Add (more) explicit statement about DTLS support to the README

[boaks]

Trying to analyze issue #661 it seems for me, that there are expectations, which are different to the current implementation.

For me, it seems, that the lwm2m server and bootstrap server don't support DTLS. it's also not mentioned in the README, that these servers are supporting DTLS. Maybe adding an explicit statement helps to overcome wrong assumptions.

So:
Is that right, the servers don't support DTLS (for now)?
If so, would adding that to the README will be OK?

[eabase]

Not sure if this project is abandoned, but I would like to know the same thing.