docs: add comments around how to set up env.config.js and testing with Jest

example.env.config.js

@@ -1,3 +1,18 @@
+/*
+Learner Dashboard is now able to handle JS-based configuration!
+
+Until a later release, the .env.* files will still be included in this repo.
+
+Note: having both .env and env.config.js files will follow a predictable order
+frontend-platform's getConfig loads configuration:
+- .env file config
+- optional handlers (commonly used to merge MFE-specific config in via additional process.env variables)
+- env.config.js file config
+- runtime config
+
+Any env vars needed for Jest Snapshot to pass can be mocked in ./src/setupTest.jsx
+*/
+
 module.exports = {
   NODE_ENV: 'development',
   NODE_PATH: './src',

src/setupTest.jsx

@@ -67,6 +67,23 @@ jest.mock('@edx/frontend-platform/i18n', () => {
   };
 });
 
+/*
+When .env.test is removed, uncomment the env vars below and add any environment variables for testing with Jest
+
+Context: Snapshot is not currently not set up to be able to parse the environment variables in env.config.js
+*/
+
+// jest.mock('@edx/frontend-platform', () => ({
+//   getConfig: jest.fn(() => ({
+//     LMS_BASE_URL: 'http://localhost:18000',
+//     LOGOUT_URL: 'http://localhost:18000/logout',
+//     LOGO_URL: 'https://edx-cdn.org/v3/default/logo.svg',
+//     MARKETING_SITE_BASE_URL: 'http://localhost:18000',
+//     SUPPORT_URL: 'http://localhost:18000/support',
+//     OPTIMIZELY_FULL_STACK_SDK_KEY: 'SDK Key',
+//   })),
+// }));
+
 jest.mock('@edx/paragon', () => jest.requireActual('testUtils').mockNestedComponents({
   Alert: {
     Heading: 'Alert.Heading',