Add "Build an LDK Light Client" Guide

[ariard]

This one is up to me to get it started, detailing how to build a lightclient with LDK and what options you're offered.

[ariard]

Features to document :

• zero-conf channel
• 0-amount invoices
• spontaenous payment
• custom network messages

[TheBlueMatt]

May be worth mentioning that the router/channels stuff is very separate and you can fetch the routing table and calculate routes without a ChannelManager at all.