docs: rfc 0250_Covenants (#3574)

Description
---
Adds RFC for covenants on Tari

[Rendered](https://demo.hedgedoc.org/s/Vwn0ZtWsS)

Motivation and Context
---
Allows sidechain checkpointing transactions and many other use-cases.

RFC/src/BaseLayerExtensions.md

@@ -5,5 +5,7 @@ Tari Base layer token and Digital Asset network.
 
 * [RFC-0220: Asset Checkpoints](RFC-0220_AssetCheckpoints.md)
 * [RFC-0230: Time related transactions](RFC-0230_HTLC.md)
+* [RFC-0201: TariScript](RFC-0201_TariScript.md)
+* [RFC-0250: Covenants](RFC-0250_Covenants.md)
 * [RFC-0322: Validator Node Registration](RFC-0322_VNRegistration.md)
-* [RFC-0322: Asset Registration](RFC-0341_AssetRegistration.md)
+* [RFC-0341: Asset Registration](RFC-0341_AssetRegistration.md)