Update README.md

README.md

@@ -6,3 +6,44 @@
 | ------- | ----------- |
 | [@actions/exit](packages/exit) | Provides utilities for exiting from an action |
 | [@actions/toolkit](packages/toolkit) | A general-purpose toolkit for writing actions |
+
+## Development
+
+This repository uses [Lerna](https://github.com/lerna/lerna#readme) to manage multiple packages. Read the documentation there to begin contributing.
+
+### Useful Scripts
+
+- `npm run bootstrap` This runs `lerna bootstrap` which will install dependencies in this repository's packages and cross-link packages where necessary.
+- `npm run build` This compiles TypeScript code in each package (this is especially important if one package relies on changes in another when you're running tests). This is just an alias for `lerna run tsc`.
+- `npm run format` This checks that formatting has been applied with Prettier.
+- `npm test` This runs all Jest tests in all packages in this repository.
+  - If you need to run tests for only one package, you can pass normal Jest CLI options:
+    ```console
+    $ npm test -- packages/toolkit
+    ```
+
+### Creating a Package
+
+1. In a new branch, create a new Lerna package:
+
+```console
+$ lerna create @actions/new-package
+```
+
+This will ask you some questions about the new package. Start with `0.0.0` as the first version (look generally at some of the other packages for how the package.json is structured).
+
+2. Add a `tsconfig.json` file for the new package (you can probably just reuse one from another existing package):
+
+```console
+$ cp packages/toolkit/tsconfig.json packages/new-package/tsconfig.json
+```
+
+3. Add `tsc` script to the new package's package.json file:
+
+```json
+"scripts": {
+  "tsc": "tsc"
+}
+```
+
+4. Start developing 😄