add some instructions to use pip install

.gitignore

@@ -2,6 +2,8 @@
 /cpp/build/
 /python/petsird/
 __pycache__/
+build/
+petsird.egg-info/
 
 # Common editor backups
 /.vscode/

python/README.md

@@ -1,6 +1,33 @@
 # PETSIRD basic Python example
 
-This directory contains some Python example code to read/write PETSIRD data. You need to `yardl generate` in the `model` directory first.
+This directory contains some Python example code to read/write PETSIRD data.
+
+## Pre-installation
+
+You will need the Python files generated by yardl. You have a few options.
+
+### Use PyPI
+
+```sh
+python3 -m venv ~/petsirdenv
+source ~/petsirdenv/bin/activate
+pip install petsird
+```
+
+### Use the model files from your local clone
+
+(This option needs the `yardl` binary, either download it, or use our devcontainer or GitHub CodeSpace.)
+
+You need to `yardl generate` in the `model` directory first. This will create the Python package files in a `petsird`
+subfolder. Optionally install those into your venv
+```sh
+python3 -m venv ~/petsirdenv
+source ~/petsirdenv/bin/activate
+cd python
+pip install .
+```
+
+## Usage
 
 The Python code shows piping the compact binary format to standard out and
 reading it from standard in. This can be used as follows: