blip2: add advisory for invoice feature bit maximum (#24)

Author: carlaKC

blip-0002.md

@@ -40,6 +40,7 @@ network split.
 Feature bits are specified in [Bolt 9](https://github.com/lightning/bolts/blob/master/09-features.md).
 They let nodes publicly advertise that they support or require a given feature.
 Feature bits in the `0`-`255` range are reserved for BOLTs: bLIPs must use feature bits above that range.
+Custom feature bits used in the `I` [Bolt 11](https://github.com/lightning/bolts/blob/master/11-payment-encoding.md) context must be <= 5114 due to the [size limitations on tagged fields](https://github.com/lightning/bolts/blob/master/11-payment-encoding.md#tagged-fields).
 
 bLIPs may reserve feature bits by adding them to the following table: