Updated batchmap docs (#3649)

I've just set this up on a new machine and confirmed it works. I've included the steps I took here because this is likely to work for other people, and it's all a bit mysterious without a script to follow.

Future work will involve updating this demo to actually explain what it is doing, because it's only just accessible to me, and I wrote it.

experimental/batchmap/README.md

@@ -51,18 +51,40 @@ are identified.
 
 ## Running the demo
 
-### Building the map
+### Setup
+
+The map uses Apache Beam to assemble and run a pipeline of tasks to construct the map.
+The following instructions show how to run this code locally using the Python portable runner.
+Comprehensive documentation for setting up a Python environment is beyond the scope of this demo, but instructions can be found at [Beam Python Tips](https://cwiki.apache.org/confluence/display/BEAM/Python+Tips).
+Steps that worked for me using python 3.10.12 on a Debian machine are:
+
+```shell
+python3 --version # Confirm 3.10.12
+sudo apt install python3-venv
+
+# Create a virtual python env for configuring the runner and deps
+python3 -m venv ~/Virtualenvs/beampy31012
+source ~/Virtualenvs/beampy31012/bin/activate
+python3 -m pip install --upgrade pip
+pip install wheel
+pip install apache-beam
+
+# Now run the portable runner, and leave it listening for work on 8099
+python -m apache_beam.runners.portability.local_job_service_main --port=8099
+```
 
-The following instructions show how to run this code locally using the Python
-portable runner. Setting up a Python environment is beyond the scope of this
-demo, but instructions can be found at [Beam Python Tips](https://cwiki.apache.org/confluence/display/BEAM/Python+Tips).
-These instructions are for Linux/MacOS but can likely be adapted for Windows.
+Alternate steps that may work from checking out the source repo:
 
-Ensure that the Python runner is running:
 1. Check out `github.com/apache/beam` (tested at branch `release-2.24.0`)
 2. `cd sdks/python` within that repository
 3. `python -m apache_beam.runners.portability.local_job_service_main --port=8099`
 
+
+### Building the map
+
+These instructions are for Linux/MacOS but can likely be adapted for Windows.
+Ensure that you have a python portable runner listening on port 8099, as documented above.
+
 In another terminal:
 1. Check out this repository and `cd` to the folder containing this README
 2. `go run ./cmd/build/mapdemo.go --output=/tmp/mapv1 --runner=universal --endpoint=localhost:8099 --environment_type=LOOPBACK`