Add River and River Lightning LNURL support to README.md

[joshfix]

River has recently introduced support for several LUDs to our brokerage and River Lightning services. Specifically, we have added:

River Brokerage & River Lightning:

• Paying to LNURL/lightning addresses (wallet-side LUD-06 and LUD-16)

River Lightning:

• Receiving to LNURL/lightning addresses (service-side LUD-06 and LUD-16)
• Withdrawing to LNURL (service-side LUD-03 and LUD-17)

This MR adds documentation to the README.md file to signal support for these features.

[SachinMeier]

tACK

[hsjoberg]

Great stuff