Add mocks section to contributing document

.github/CONTRIBUTING.md

@@ -27,6 +27,40 @@ guidelines:
 - Commit messages should be prefixed with the package(s) they modify.
   - E.g. "eth, rpc: make trace configs optional"
 
+### 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/mockgen@v0.4` to generate a mock for your interface (preferred); or
+    - add another `//go:generate go run go.uber.org/mock/mockgen@v0.4` 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/mockgen@v0.4 -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.
+
 ## Documentation guidelines
 
 - Code should be well commented, so it is easier to read and maintain.
@@ -35,7 +69,7 @@ guidelines:
   [commentary](https://go.dev/doc/effective_go#commentary) guidelines.
 - Changes with user facing impact (e.g., addition or modification of flags and
  options) should be accompanied by a link to a pull request to the [avalanche-docs](https://github.com/ava-labs/avalanche-docs)
- repository. [example](https://github.com/ava-labs/avalanche-docs/pull/1119/files). 
+ repository. [example](https://github.com/ava-labs/avalanche-docs/pull/1119/files).
 - Changes that modify a package significantly or add new features should
  either update the existing or include a new `README.md` file in that package.