Notes on latest tailscale (1.68.2)

[BeautyyuYanli]

Seems that the latest version of tailscale/derp relays on STUN for latency detection, so:

• STUN must be enabled
• network_mode should be set to "host"

I'm using Aliyun ECS (99/year) to run derp, and I've been struggling with this problem for several weeks :(

If more people run into the same issue, I believe these notes should be add to readme.

[BeautyyuYanli]

The second note seems to be related to #5