Add instruction for conformance testing against local build (#4910)

* Add instructions for conformance testing against local build with examples

* Add instructions to build the Haskell code from formal ledger

---------

Co-authored-by: teodanciu <teodora.danciu@tweag.io>

Author: carlostome

CONTRIBUTING.md

@@ -289,6 +289,12 @@ from [the Shelley ledger spec](./eras/shelley/formal-spec)).
 4. Add a link to the package near the bottom of [flake.nix](./flake.nix),
    following the existing examples.
 
+### To build the Haskell code from the Agda ledger spec
+
+See [Build the spec using nix-build](https://github.com/IntersectMBO/formal-ledger-specifications/tree/master?tab=readme-ov-file#build-the-spec-using-nix-build)
+and [Contributing](https://github.com/IntersectMBO/formal-ledger-specifications/blob/master/CONTRIBUTING.md)
+in the [formal-ledger-specifications repo](https://github.com/IntersectMBO/formal-ledger-specifications).
+
 ### To update the referenced Agda ledger spec
 
 To update the version of the Agda spec that the conformance tests are using:
@@ -304,6 +310,41 @@ To update the version of the Agda spec that the conformance tests are using:
 
 If the commit you need in `formal-ledger-specifications` is not on master, open a PR for your branch in the `formal-ledger-specifications` repository. This will create a branch with the updated generated code, which you can then use as described above. You will not be able to merge in `cardano-leder` master a reference to a commit not yet merged in `formal-ledger-specifications`.
 
+### To run conformance testing against a local build of the Agda ledger spec
+
+1. Enter the `nix develop` shell
+2. Comment out the `source-repository-package` section in `cabal.project`
+3. Add the path to the local build directory (containing the `cardano-ledger-executable-spec.cabal` file, see below for examples) to `packages` in `cabal.project`
+4. Execute the tests, e.g., running `cabal test cardano-ledger-conformance`
+
+To change the local build directory, redo step 3 _without leaving_ the `nix develop` shell.
+
+#### Examples
+
+- As an absolute path:
+
+    Compiling the `formal-ledger-specifications` to Haskell via:
+    ```shell
+    nix-build -A ledger.hsSrc
+    ```
+    returns a path to the nix store, e.g., `/nix/store/9pv3x44dfnwrz0jjrh9mlxa9y143i987-hs-src-0.1`, that can be used as:
+    ```
+    /nix/store/9pv3x44dfnwrz0jjrh9mlxa9y143i987-hs-src-0.1/haskell/Ledger
+    ```
+    in the packages section of the `cabal.project` file.
+
+- As a relative path:
+
+    Cloning the [formal-ledger-specifications repo](https://github.com/IntersectMBO/formal-ledger-specifications) on a `MAlonzo` branch as a sibling folder, e.g., `spec`, of `cardano-leder`:
+    ```shell
+    git clone -b MAlonzo-code git@github.com:IntersectMBO/formal-ledger-specifications.git spec
+    ```
+    and using:
+    ```
+    ../spec/generated/
+    ```
+    in the `cabal.project` file.
+
 ### Additional documentation
 
 You can find additional documentation on the nix infrastructure used in this