Add motivation paragraph in introduction

[huitema]

From the logs of the Zulip chatroom:

From	Text
Martin Thomson	That STUN argument is not a good one. Is there a better argument for using QUIC? It seems like there might be.
Martin Thomson	That STUN argument refers to the argument that Marten used to argue against using STUN.
Christian Huitema	Martin: I like that we can have address discovery potentially with ANY server. It is good for P2P, because of "don't stick out". But it can also be used for debug, etc.
Martin Thomson	Christian: that's a good one.
Martin Thomson	I think that the authentication in STUN is also particularly complex for this purpose.
Martin Thomson	STUN relies on shared keys, which would mean the use of exporters or something like that.
Christian Huitema	Yes, having everything encrypted is a also a huge plus.

[marten-seemann]

@huitema Can you propose some text?

[huitema]

Will do.