Add specification link
* Add specification link
Added Fuel ABI specification links to the calling contracts section
* Update docs/src/blockchain-development/calling_contracts.md
Smart contracts can be _called_ by other contracts or scripts. In the FuelVM, this is done primarily with the [`call`](https://github.com/FuelLabs/fuel-specs/blob/master/specs/vm/opcodes.md#call-call-contract) instruction.
-Sway provides a nice way to manage callable interfaces with its `abi` system.
+Sway provides a nice way to manage callable interfaces with its `abi` system. The Fuel ABI specification can be found [here](https://github.com/FuelLabs/fuel-specs/blob/master/specs/protocol/abi.md).
## Example
