docs(config): basic usage and config setup

README.md

@@ -1,3 +1,95 @@
 # Littlepay
 
 APIs and admin tasks for Littlepay.
+
+## Usage
+
+```console
+$ littlepay -h
+usage: littlepay [-h] [-v] {config,switch} ...
+
+positional arguments:
+  {config,switch}
+    config         Get or set configuration.
+    switch         Switch the active environment or participant.
+
+options:
+  -h, --help       show this help message and exit
+  -v, --version    show program's version number and exit
+```
+
+## Install
+
+Use `pip` to install from GitHub:
+
+```console
+pip install git+https://github.com/cal-itp/littlepay.git@main"
+```
+
+## Getting started
+
+If this is your first time using `littlepay`, create a configuration file (using defaults):
+
+```console
+littlepay config
+```
+
+The location and basic info from your current config file are printed in the terminal:
+
+```
+$ littlepay config
+Creating config file: /home/calitp/.littlepay/config.yaml
+Config: /home/calitp/.littlepay/config.yaml
+Envs: prod, qa
+Participants: cst
+Active: qa, [no participant]
+```
+
+### Add configuration
+
+The configuration file is a YAML dictionary tracking `envs` and `participants`:
+
+```yml
+active:
+  env: qa
+  participant: ""
+envs:
+  prod:
+    url: ""
+  qa:
+    url: ""
+participants:
+  cst:
+    audience: ""
+    client_id: ""
+    client_secret: ""
+```
+
+There are two `envs` by default, the base API URL should be completed for each:
+
+- `qa` is for the testing system
+- `prod` is for the live system
+
+Add specifics for the `participants` you manage based on information received from Littlepay support.
+
+### Use a different config file
+
+```console
+littlepay config -c /path/to/new/config.yaml
+```
+
+The most recent config used is saved for next time.
+
+### Switch the active target
+
+For `envs`:
+
+```console
+littlepay switch env <env_name>
+```
+
+And `participants`:
+
+```console
+littlepay switch participant <participant_id>
+```