document Go patterns used in the codebase

docs/go.md

@@ -0,0 +1,42 @@
+# Go
+
+## Patterns
+
+- To ensure interface cannot be implemented in other packages,
+  add a private function (first character must be lower-case) named "is" + type name,
+  which takes no arguments, returns nothing, and has an empty body.
+
+  For example:
+
+  ```go
+  type I interface {
+      isI()
+  }
+  ```
+
+  See https://go.dev/doc/faq#guarantee_satisfies_interface
+
+- To ensure a type implements an interface at compile-time,
+  use the "interface guard" pattern:
+  Introduce a global variable named `_`, type it as the interface,
+  and assign an empty value of the concrete type to it.
+
+  For example:
+
+  ```go
+  type T struct {
+      //...
+  }
+
+  var _ io.ReadWriter = (*T)(nil)
+
+  func (t *T) Read(p []byte) (n int, err error) {
+  // ...
+  ```
+
+  See
+  - https://go.dev/doc/faq#guarantee_satisfies_interface
+  - https://rednafi.com/go/interface_guards/
+  - https://github.com/uber-go/guide/blob/master/style.md#verify-interface-compliance
+  - https://medium.com/@matryer/golang-tip-compile-time-checks-to-ensure-your-type-satisfies-an-interface-c167afed3aae
+