Add environment variable documentation
Adds documentation for the environment variables defined in the
Splinter example docker-compose file.
@@ -186,6 +186,26 @@ circuit is created.
## Demonstrate Grid Smart Contract Functionality
+**Note:** To simplify this procedure, the example `docker-compose.yaml` file
+defines environment variables for the ``gridd-alpha`` and ``gridd-beta``
+containers. These variables define the Grid daemon's key file and endpoint,
+so you don't have to use the `-k` and `--url` options with the `grid` command in
+this section.
+
+The following environment variables apply only to this example. If you want to
+override these values, you can edit `docker-compose.yaml` to redefine the
+variables, or you can use the associated option with the `grid` commands in
+steps 3 through 10.
+
+- `GRID_DAEMON_KEY` defines the key file name for the Grid daemon, as generated
+ by the `docker-compose.yaml` file. Use `-k <keyfile>` to override this
+ variable on the command line.
+
+- `GRID_DAEMON_ENDPOINT` defines the endpoint for the ``gridd-alpha`` or
+ ``gridd-beta`` container. Use `--url <endpoint>` to override this variable
+ on the command line.
+
+
1. Start a bash session in the `gridd-alpha` Docker container. You will use
this container to run Grid commands on `alpha-node-000`.
