add readme

composer/README.md

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+# Testground Composer
+
+This is a work-in-progress UI for configuring and running testground compositions.
+
+The app code lives in [./app](./app), and there's a thin Jupyter notebook shell in [composer.ipynb](./composer.ipynb).
+
+## Running
+
+You can either run the app in docker, or in a local python virtualenv. Docker is recommended unless you're hacking
+on the code for Composer itself.
+
+### Running with docker
+
+Run the `./composer.sh` script to build a container with the latest source and run it. The first build
+will take a little while since it needs to build testground and fetch a bunch of python dependencies.
+
+You can skip the build if you set `SKIP_BUILD=true` when running `composer.sh`, and you can rebuild
+manually with `make docker`.
+
+The contents of `$TESTGROUND_HOME/plans` will be sync'd to a temporary directory and read-only mounted 
+into the container.
+
+After building and starting the container, the script will open a browser to the composer UI.
+
+You should be able to load an existing composition or create a new one from one of the plans in 
+`$TESTGROUND_HOME/plans`.
+
+Right now docker only supports the standalone webapp UI; to run the UI in a Jupyter notebook, see below.
+
+### Running with local python
+
+To run without docker, make a python3 virtual environment somewhere and activate it:
+
+```shell
+# make a virtualenv called "venv" in the current directory
+python3 -m venv ./venv
+
+# activate (bash/zsh):
+source ./venv/bin/activate
+
+# activate (fish):
+source ./venv/bin/activate.fish
+```
+
+Then install the python dependencies:
+
+```shell
+pip install -r requirements.txt
+```
+
+And start the UI:
+
+```shell
+panel serve composer.ipynb
+```
+
+That will start the standalone webapp UI. If you want a Jupyter notebook instead, run:
+
+```
+jupyter notebook
+```
+
+and open `composer.ipynb` in the Jupyter file picker.