Doc/update receiver docs

pallets/fulfillment-receiver/README.md

@@ -287,3 +287,87 @@ mod receiver {
     }
 }
 ```
+
+## WASM Smart Contract direct integration
+
+It is also possible to directly fulfill to a WASM smart contract without goign through a pallet first. For example, given the following smart contract:
+
+```rust
+#![cfg_attr(not(feature = "std"), no_std)]
+
+use ink;
+
+#[ink::contract]
+mod receiver {
+
+    use ink::{
+        env::{caller, DefaultEnvironment},
+        storage::Mapping,
+    };
+
+    #[ink(storage)]
+    pub struct Receiver {
+        allowed_processors: Mapping<AccountId, ()>,
+        price: u128,
+    }
+
+    impl Receiver {
+
+        #[ink(constructor)]
+        pub fn default() -> Self {
+            Self {
+                allowed_processors: Default::default(),
+                price: Default::default(),
+            }
+        }
+
+        #[ink(message)]
+        pub fn fulfill(&mut self, price: u128) {
+            let caller = caller::<DefaultEnvironment>();
+            if self.allowed_processors.contains(&caller) {
+                self.price = price;
+            }
+        }
+
+        #[ink(message)]
+        pub fn get_price(&self) -> u128 {
+            self.price
+        }
+
+        #[ink(message)]
+        pub fn add_processor(&mut self, processor: AccountId) {
+            self.allowed_processors.insert(processor, &());
+        }
+
+        #[ink(message)]
+        pub fn remove_processor(&mut self, processor: AccountId) {
+            self.allowed_processors.remove(&processor);
+        }
+    }
+}
+```
+
+It is possible to directly call the fulfill method directly from the processors. The example script below shows how to call the above smart contract deployed on the Shibuya parachain:
+
+```javascript
+const callIndex = '0x4606'; // the call index for the "call" extrinsic of pallet-contracts
+const payload = _STD_.chains.substrate.codec.encodeUnsignedNumber(2, 128); // encoding the price value (2) as an u128
+const destination = "XTj3CLB3G6WnwPMgYo1PM2xi9BkDegtGw5WPd7X36G515La"; // the smart contract address
+_STD_.chains.substrate.signer.setSigner("SECP256K1"); // select which curve to use for the signature
+_STD_.chains.substrate.contract.fulfill(
+    "https://shibuya-rpc.dwellir.com", // the parachain rpc endpoint
+    callIndex,
+    destination,
+    payload,
+    {
+        refTime: "3951114240",
+        proofSize: "629760",
+    },
+    (opHash) => {
+        print("Succeeded: " + opHash)
+    },
+    (err) => {
+        print("Failed: " + err)
+    },
+)
+```