more verbose readme

README.md

@@ -1,6 +1,6 @@
 # [CLI Utility] Google Play App Version Code
 
-Retrieve the latest (or next) version code for a Google Play App. Useful for automating build scripts.
+Retrieve the latest (or next) [versionCode](https://developer.android.com/studio/publish/versioning#versioningsettings) for a Google Play App. Useful for automating build scripts.
 
 ## Setup
 
@@ -40,3 +40,55 @@ sudo apt-get install ./google-app-version.deb
 # (Optional) Add the Release to Path (to simply call `google-app-version` from any working directory)
 sudo ln -s /usr/local/bin/google-app-version/bin/google-app-version /usr/bin
 ```
+
+## Usage
+
+- The CLI supports two output modes:
+
+  1. the `latest`/current `versionCode` of an application
+  2. the `next` `versionCode` of an application (just `latest` + 1)
+
+- The contents of the service account's `key.json` file should be passed as an environment variable (useful for CI/CD environments)
+
+### Example
+
+For an application with a production build versionCode of 14:
+
+#### Latest
+
+```shell
+JSON=$(cat ~/key.json) && google-app-version latest -f "$JSON" -p com.company.app
+# 14
+```
+
+#### Next
+
+```shell
+JSON=$(cat ~/path/to/key.json) && google-app-version next -f "$JSON" -p com.company.app
+# 15
+```
+
+### Parameters
+
+- Version (Position 0): `google-app-version <VERSION>`
+  - `latest`: the latest, currently released `versionCode` of an application
+    - `google-app-version latest ...`
+  - `next`: the next-available `versionCode` of an application
+    - `google-app-version next ...`
+
+### Required Arguments
+
+- JSON File Contents (`-f` / `--file`): the **contents** (not filepath) of a JSON file for the google service account's credentials
+  - Note: if the file's contents are stored locally, instead of in an environment variable, you can use the following command structure to load the JSON file's contents into an environment variable:
+    - `MY_JSON_FILE_CONTENTS=$(cat ~/path/to/key.json) && google-app-version -f "$MY_JSON_FILE_CONTENTS" ...`
+- Package Name (`-p` / `--package`): the **package** (not application) name
+  - this is set the after your first release
+  - in the below example image, (Google Play Console > Home > Apps) the package name is `com.inspiretmstech.mobile`
+
+![](./docs/package-name.png)
+
+### Optional Arguments
+
+- Verbose Output (`-v` / `--verbose`): print debug-level information to the console
+- Help (`-h` / `--help`): print full documentation and exit
+- Version (`-V` / `--version`): print the CLI version and exit