Document how to use a custom tezos build (#430)

README.md

@@ -19,6 +19,7 @@
   - [Adding external nodes to the cluster](#adding-external-nodes-to-the-cluster)
     - [On the computer of the joining node](#on-the-computer-of-the-joining-node)
   - [RPC Authentication](#rpc-authentication)
+- [Using a custom Tezos build](#using-a-custom-tezos-build)
 - [Indexers](#indexers)
 - [Notes](#notes)
 - [Development](#development)
@@ -423,6 +424,41 @@ You can optionally spin up an RPC authentication backend allowing trusted users
 
 Follow the steps [here](./rpc-auth/README.md).
 
+# Using a custom Tezos build
+
+Create a clone of the `[tezos](https://gitlab.com/tezos/tezos)`
+repository.  [Set up your development environment as usual](https://tezos.gitlab.io/introduction/howtoget.html#setting-up-the-development-environment-from-scratch).  Then run:
+
+```shell
+eval $(minikube docker-env)
+make docker-image
+```
+
+This will create a docker image called `tezos:latest` and install it
+into the minikube environment.
+
+Or, if you prefer, you can build the image using:
+```shell
+./scripts/create_docker_image.sh
+```
+
+This will create an image with a name like `tezos/tezos:v13-rc1`.
+Then you install it thus:
+```shell
+docker image save <image> | ( eval $(minikube docker-env); docker image load )
+```
+
+Either way, inside `$CHAIN_NAME_values.yaml`, change the `images` section to:
+
+```yaml
+images:
+  octez: <image>
+```
+
+where image is `tezos:latest` or whatever.
+
+Then install the chart as above.
+
 # Indexers
 
 You can optionally spin up a Tezos blockchain indexer that makes querying for information very quick. An indexer puts the chain contents in a database for efficient indexing. Most dapps need it. You can read more about indexers [here](https://wiki.tezosagora.org/build/blockchain-indexers).