chore(internal): Update SETUP.md (#5530)

Update SETUP.md

SETUP.md

@@ -1,7 +1,29 @@
-## Install required packages
+# Setup
 
-1. Install [nvm](https://github.com/nvm-sh/nvm#install--update-script)
-2. Clone repo
-3. `nvm use 18`
-4. `npm install --global yarn`
-5. `yarn`
+The Fern repo is primarily written in TypeScript and relies on [PnPm](https://pnpm.io/) for package management.
+
+Once you have cloned or forked the repository, run through the steps below.
+
+### Step 1: Install dependencies
+
+```sh
+pnpm install
+```
+
+### Step 2: Compile
+
+To compile all the packages in this monorepo, run `pnpm compile`.
+
+To compile a single package, filter to the relevant package: `pnpm --filter @fern-api/openapi-parser compile`.
+
+### Step 3: Testing
+
+This repo contains both unit tests and integration (end-to-end) tests.
+
+To run all the unit tests: `pnpm test`.
+
+To run unit tests for a single package: `pnpm --filter @fern-api/openapi-parser test`
+
+To run the integration tests: `pnpm test:ete`.
+
+Many of our tests rely on [snapshot testing](https://jestjs.io/docs/snapshot-testing). To rewrite snapshots, use `pnpm test:update` or `pnpm test:ete:update`.