message: merge again the rfcs for ffadd (old variant)
diff: - [RFC 2: Maller's optimization for kimchi](./plonk/maller_15.md)
  - [RFC 3: Plookup integration in kimchi](./rfcs/3-lookup.md)
  - [RFC 4: Foreign Field Addition](./rfcs/ffadd.md)
  --
  +
  # Specifications
  - [Poseidon hash](./specs/poseidon.md)
  