adding a readme

README.md

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+# @embroider/try
+
+A tiny utility to support matrix testing.
+
+## List
+
+`pnpm -s @embroider/try list`
+
+Processes your `.try.mjs` file and emits JSON that can be used in GitHub Actions to create your `strategy.matrix`.
+
+## Apply
+
+`pnpm dlx @embroider/try apply $SCENARIO_NAME`
+
+Applies a scenario to your project by editing package.json, etc. Use this before the regular `pnpm install` or `npm install` in order to test differing dependencies.
+
+## Config File: .try.mjs
+
+```ts
+export default interface {
+  scenarios: Scenario[];
+}
+interface Scenario {
+  name: string;
+
+  // Values that will be merged into package.json:
+  //
+  //   npm: {
+  //     devDependencies: {
+  //       'ember-source': '^6.7.8'
+  //     }
+  //   }
+  npm?: {
+    devDependencies:  Record<string, string>;
+    dependencies:     Record<string, string>;
+    peerDependencies: Record<string, string>;
+    overrides:        Record<string, string>;
+  },
+
+  // Files that will be (over)written with the given contents
+  //
+  //   files: {
+  //     'tsconfig.json': '{ compilerOptions: {} }'
+  //   }
+  files?: Record<string, string>,
+}
+```