go: Add documentation for golang interfaces (#1332)

* Add documentation for golang interfaces

* Update go.md

Co-authored-by: Rico Sta. Cruz <rstacruz@users.noreply.github.com>

go.md

@@ -581,6 +581,50 @@ By defining your receiver as a pointer (`*Vertex`), you can do mutations.
 
 See: [Pointer receivers](https://tour.golang.org/methods/4)
 
+## Interfaces
+
+### A basic interface
+
+```go
+type Shape interface {
+  Area() float64
+  Perimeter() float64
+}
+```
+
+### Struct
+
+```go
+type Rectangle struct {
+  Length, Width float64
+}
+```
+
+Struct `Rectangle` implicitly implements interface `Shape` by implementing all of its methods.
+
+### Methods
+
+```go
+func (r Rectangle) Area() float64 {
+  return r.Length * r.Width
+}
+
+func (r Rectangle) Perimeter() float64 {
+  return 2 * (r.Length + r.Width)
+}
+```
+
+The methods defined in `Shape` are implemented in `Rectangle`.
+
+### Interface example
+
+```go
+func main() {
+  var r Shape = Rectangle{Length: 3, Width: 4}
+  fmt.Printf("Type of r: %T, Area: %v, Perimeter: %v.", r, r.Area(), r.Perimeter())
+}
+```
+
 ## References
 
 ### Official resources