test: add example of mocking external types

I realised that when I switched to using `packages.Load()` I actually added the ability to mock external types (i.e. types that aren't part of your own source tree). I've just added an example test and updated the README.

README.md

@@ -10,7 +10,6 @@ At the moment Kelpie is very much in development, and there are missing features
 
 The following is a list of known-outstanding features:
 
-- [ ] Generating mocks for external (i.e. not in your source tree) interfaces by name.
 - [ ] Generating mocks for inline interfaces in structs.
 
 ## Quickstart
@@ -206,6 +205,32 @@ t.True(mock.Called(registrationservice.Register(kelpie.Any[string]()).Times(2)))
 t.True(mock.Called(registrationservice.Register("Wendy").Never()))
 ```
 
+### Mocking an interface from an external package
+
+Kelpie can happily mock interfaces that aren't part of your own source. You don't need to do anything special to mock an "external" interface - just specify the package and interface name you want to mock:
+
+```go
+//go:generate go run ../cmd/kelpie generate --package io --interfaces Reader
+
+func (t *ExternalTypesTests) Test_CanMockAnExternalType() {
+	// Arrange
+	var bytesRead []byte
+	mock := reader.NewMock()
+	mock.Setup(reader.Read(kelpie.Match(func(b []byte) bool {
+		bytesRead = b
+		return true
+	})).Return(20, nil))
+
+	// Act
+	read, err := mock.Instance().Read([]byte("Hello World!"))
+
+	// Assert
+	t.NoError(err)
+	t.Equal(20, read)
+	t.Equal([]byte("Hello World!"), bytesRead)
+}
+```
+
 ## FAQ
 
 ### What makes Kelpie so magical

examples/external_types_test.go

@@ -0,0 +1,37 @@
+package examples
+
+import (
+	"testing"
+
+	"github.com/adamconnelly/kelpie"
+	"github.com/adamconnelly/kelpie/examples/mocks/reader"
+	"github.com/stretchr/testify/suite"
+)
+
+//go:generate go run ../cmd/kelpie generate --package io --interfaces Reader
+
+type ExternalTypesTests struct {
+	suite.Suite
+}
+
+func (t *ExternalTypesTests) Test_CanMockAnExternalType() {
+	// Arrange
+	var bytesRead []byte
+	mock := reader.NewMock()
+	mock.Setup(reader.Read(kelpie.Match(func(b []byte) bool {
+		bytesRead = b
+		return true
+	})).Return(20, nil))
+
+	// Act
+	read, err := mock.Instance().Read([]byte("Hello World!"))
+
+	// Assert
+	t.NoError(err)
+	t.Equal(20, read)
+	t.Equal([]byte("Hello World!"), bytesRead)
+}
+
+func TestExternalTypes(t *testing.T) {
+	suite.Run(t, new(ExternalTypesTests))
+}