docs: how to merge from public upstream (#40)

* docs: how to merge from public upstream

* update contents

README.md

@@ -18,7 +18,6 @@ is expected to be used by Storage library maintainers.
     - [Run the testbench](#run-the-testbench)
     - [Start the gRPC server](#start-the-gRPC-server)
     - [Check that the testbench is running](#check-that-the-testbench-is-running)
-  - [Updating Proto Files](#updating-proto-files)
   - [Force Failures](#force-failures)
     - [return-broken-stream](#return-broken-stream)
     - [return-corrupted-data](#return-corrupted-data)
@@ -35,6 +34,8 @@ is expected to be used by Storage library maintainers.
     - [Delete a Retry Test resource](#delete-a-retry-test-resource)
     - [Causing a failure using x-retry-test-id header](#causing-a-failure-using-x-retry-test-id-header)
     - [Forced Failures Supported](#forced-failures-supported)
+  - [Merging from Upstream](#merging-from-upstream)
+  - [Updating Proto Files](#updating-proto-files)
   - [Releasing the testbench](#releasing-the-testbench)
 
 ## Issue Policy
@@ -120,24 +121,6 @@ The response you get should be: `OK`
 
 Now you can use the testbench (while it's running) with the client libraries.
 
-## Updating Proto Files
-
-From time to time you may need to update the files generated by protobuf and
-gRPC. To do so, clone the [protos](https://github.com/googleapis/googleapis) and
-run the grpc_tools generator:
-
-```shell
-cd $HOME/storage-testbench
-pip install --no-deps grpcio-tools
-./update-protos.sh $PWD/.googleapis
-```
-
-Then commit the files generated in `google/**`:
-
-```shell
-git commit -m"chore: update protos" google
-```
-
 ## Force Failures
 
 You can force the following failures by using the `x-goog-emulator-instructions` header.
@@ -276,6 +259,78 @@ curl -H "x-retry-test-id: 1d05c20627844214a9ff7cbcf696317d" "http://localhost:91
 | redirect-send-token-T                     | [HTTP] Unsupported [GRPC] Testbench will fail the RPC with `ABORTED` and include appropriate redirection error details.
 | redirect-send-handle-and-token-T          | [HTTP] Unsupported [GRPC] Testbench will fail the RPC with `ABORTED` and include appropriate redirection error details.
 
+## How-to Guide: maintaining the pre-launch branch(es)
+
+### Merging from Upstream
+
+Clone a repository (you may already have one) that has both the public and
+`*-prelaunch` repos as remotes. We choose a configuration that makes it really
+hard to accidentally push prelaunch code to the public repo:
+
+```shell
+git clone git@github.com:googleapis/storage-testbench.git
+cd storage-testbench
+git remote add prelaunch git@github.com:googleapis/storage-testbench-prelaunch.git
+```
+
+Update your clone to the latest versions:
+
+```shell
+git fetch prelaunch
+git fetch origin
+```
+
+Find out the latest commit from the public repo already present in the
+`*-prelaunch` repo:
+
+```shell
+A=$(git rev-parse prelaunch/upstream)
+```
+
+Find out the latest commit from the public repo main branch:
+
+```shell
+B=$(git rev-parse origin/main)
+```
+
+Update the `upstream` branch on the `*-prelaunch` repository:
+
+```shell
+git push prelaunch origin/main:upstream
+```
+
+You can use a separate workspace for the next steps:
+
+```shell
+cd $HOME/
+git clone git@github.com:googleapis/storage-testbench-prelaunch.git
+cd storage-testbench
+git checkout pre-launch-acv2
+git checkout -b chore-merge-from-public-circa-$(date +%Y-%m-%d)
+git cherry-pick ${A}..${B}
+```
+
+Send a PR and squash commit as usual.
+
+
+## Updating Proto Files
+
+From time to time you may need to update the files generated by protobuf and
+gRPC. To do so, clone the [protos](https://github.com/googleapis/googleapis) and
+run the grpc_tools generator:
+
+```shell
+cd $HOME/storage-testbench
+pip install --no-deps grpcio-tools
+./update-protos.sh $PWD/.googleapis
+```
+
+Then commit the files generated in `google/**`:
+
+```shell
+git commit -m"chore: update protos" google
+```
+
 ## Releasing the testbench
 
 The repository currently uses [Github Tags](https://docs.github.com/en/repositories/releasing-projects-on-github/managing-releases-in-a-repository#creating-a-release)