Add bLIP 52: JIT Channel Negotiation (#54)

A "JIT Channel" is a channel opened in response to an incoming payment
from the public network to a client, via the LSP.
This allows a client with no Lightning channels to start receiving on
Lightning, and have the cost of their inbound liquidity be deducted from their
first received payment.

The given protocol is based on LSPS0/blip-0050 and has been previously
stabilized by the LSP Spec group and is live in production with several
LSP and client implementations today.

As previously discussed on multiple occasions, the LSP Spec
group is however moving to a bLIP-centric process, which is why we
'upstream' these previously-stabilized specifications here.

README.md

@@ -30,3 +30,4 @@ For more detail on the process, please read [bLIP-0001](./blip-0001.md) and
 | [32](./blip-0032.md)     | Onion Message DNS Resolution    | Matt Corallo                | Active |
 | [50](./blip-0050.md)     | LSPS0: LSP Spec Transport Layer | ZmnSCPxj jxPCSnmZ           | Active |
 | [51](./blip-0051.md)     | LSPS1: Channel Requests         | Severin Bühler              | Active |
+| [52](./blip-0052.md)     | LSPS2: JIT Channel Negotiation  | ZmnSCPxj jxPCSnmZ           | Active |