docs: document client ID support for app-id

Co-authored-by: parkerbxyz <17183625+parkerbxyz@users.noreply.github.com>

tests/index.js.snapshot

@@ -1,3 +1,20 @@
+exports[`main-client-id.test.js > stdout 1`] = `
+Inputs 'owner' and 'repositories' are not set. Creating token for this repository (actions/create-github-app-token).
+::add-mask::ghs_16C7e42F292c6912E7710c838347Ae178B4a
+
+::set-output name=token::ghs_16C7e42F292c6912E7710c838347Ae178B4a
+
+::set-output name=installation-id::123456
+
+::set-output name=app-slug::github-actions
+::save-state name=token::ghs_16C7e42F292c6912E7710c838347Ae178B4a
+::save-state name=expiresAt::2016-07-11T22:14:10Z
+--- REQUESTS ---
+GET /repos/actions/create-github-app-token/installation
+POST /app/installations/123456/access_tokens
+{"repositories":["create-github-app-token"]}
+`;
+
 exports[`main-custom-github-api-url.test.js > stdout 1`] = `
 Inputs 'owner' and 'repositories' are set. Creating token for the following repositories:
       

tests/main-client-id.test.js

@@ -0,0 +1,6 @@
+import { test } from "./main.js";
+
+// Verify `main` accepts a GitHub App client ID via the `app-id` input
+await test(() => {
+  process.env["INPUT_APP-ID"] = "Iv1.0123456789abcdef";
+});