Update docs

contracts/mocks/CallReceiverMock.sol

@@ -0,0 +1,50 @@
+// SPDX-License-Identifier: MIT
+
+pragma solidity >=0.6.0 <0.8.0;
+
+contract CallReceiverMock {
+    string public sharedAnswer;
+
+    event MockFunctionCalled();
+
+    uint256[] private _array;
+
+    function mockFunction() public payable returns (string memory) {
+        emit MockFunctionCalled();
+
+        return "0x1234";
+    }
+
+    function mockFunctionNonPayable() public returns (string memory) {
+        emit MockFunctionCalled();
+
+        return "0x1234";
+    }
+
+    function mockStaticFunction() public pure returns (string memory) {
+        return "0x1234";
+    }
+
+    function mockFunctionRevertsNoReason() public payable {
+        revert();
+    }
+
+    function mockFunctionRevertsReason() public payable {
+        revert("CallReceiverMock: reverting");
+    }
+
+    function mockFunctionThrows() public payable {
+        assert(false);
+    }
+
+    function mockFunctionOutOfGas() public payable {
+        for (uint256 i = 0; ; ++i) {
+            _array.push(i);
+        }
+    }
+
+    function mockFunctionWritesStorage() public returns (string memory) {
+        sharedAnswer = "42";
+        return "0x1234";
+    }
+}