docs(contributing): add mocks section

Author: qdm12

.github/CONTRIBUTING.md

@@ -33,3 +33,36 @@ Please make sure your contributions adhere to our coding guidelines:
 Before you submit a feature request, please check and make sure that it isn't 
 possible through some other means.
 
+## Mocks
+
+Mocks are auto-generated using [mockgen](https://pkg.go.dev/go.uber.org/mock/mockgen) and `//go:generate` commands in the code.
+
+* To **re-generate all mocks**, use the command below from the root of the project:
+
+    ```sh
+    go generate -run "go.uber.org/mock/mockgen" ./...
+    ```
+
+* To **add** an interface that needs a corresponding mock generated:
+  * if the file `mocks_generate_test.go` exists in the package where the interface is located, either:
+    * modify its `//go:generate go run go.uber.org/mock/[email protected]` to generate a mock for your interface (preferred); or
+    * add another `//go:generate go run go.uber.org/mock/[email protected]` to generate a mock for your interface according to specific mock generation settings
+  * if the file `mocks_generate_test.go` does not exist in the package where the interface is located, create it with content (adapt as needed):
+
+    ```go
+    // Copyright (C) 2019-2025, Ava Labs, Inc. All rights reserved.
+    // See the file LICENSE for licensing terms.
+
+    package mypackage
+
+    //go:generate go run go.uber.org/mock/[email protected] -package=${GOPACKAGE} -destination=mocks_test.go . YourInterface
+    ```
+
+    Notes:
+    1. Ideally generate all mocks to `mocks_test.go` for the package you need to use the mocks for and do not export mocks to other packages. This reduces package dependencies, reduces production code pollution and forces to have locally defined narrow interfaces.
+    1. Prefer using reflect mode to generate mocks than source mode, unless you need a mock for an unexported interface, which should be rare.
+* To **remove** an interface from having a corresponding mock generated:
+  1. Edit the `mocks_generate_test.go` file in the directory where the interface is defined
+  1. If the `//go:generate` mockgen command line:
+      * generates a mock file for multiple interfaces, remove your interface from the line
+      * generates a mock file only for the interface, remove the entire line. If the file is empty, remove `mocks_generate_test.go` as well.