Add monorepo Contributing guide (openapi-ts#1726)

Author: drwpow

CONTRIBUTING.md

@@ -0,0 +1,25 @@
+# Contributing
+
+Contributions are welcome! T
+
+## Setup
+
+This monorepo uses [pnpm workspaces](https://pnpm.io/) that lets packages pull from local versions rather than remote. After installing pnpm, running this command from the root will set everything up:
+
+```sh
+pnpm i
+```
+
+## Structure
+
+This monorepo has 2 main sections: the `docs/` that deploy to `openapi-ts.dev`, and `packages/` which are all the npm packages. Each npm package has its own `CONTRIBUTING.md` doc that describes setup needed for that package.
+
+```
+┬── docs/                         # openapi-ts.dev
+│
+└── packages/
+    ├── openapi-fetch/            # openapi-fetch package
+    ├── openapi-typescript-helpers/
+    │                             # openapi-typescript package
+    └── openapi-typescript/       # openapi-typescript package
+```