Add README, .env.example

.env.example

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+CLOUDFLARE_API_TOKEN=
+TARGET_ZONE_ID=

README.md

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 # fastmail-cloudflare
 
 This is a simple script to update the DNS records of a domain in Cloudflare to point to the Fastmail mail servers.
+
+> [!WARNING]
+> This script is not officially supported by Fastmail. Use at your own risk.
+> Furthermore, it's currently in development and may not work as expected.
+
+## Requirements
+
+- Node v20+
+  - Newer or older may work, but is untested.
+- [pnpm][pnpm]
+- Cloudflare API Token
+  - `DNS Settings:Edit` permission for the zone you want to onboard.
+  - Create one [here][cf-token].
+- The domain you want to onboard, managed by Cloudflare.
+
+## Usage
+
+1. Clone the repository
+
+```
+git clone https://github.com/Xevion/fastmail-cloudflare.git`
+```
+
+2. Install dependencies
+
+```
+cd fastmail-cloudflare && pnpm install
+```
+
+3. Configure environment variables
+
+   - Get your Cloudflare API Token.
+   - Copy `.env.example` to `.env` and fill in the values.
+   - `TARGET_ZONE_ID` is the ID of the Zone/Domain you want to onboard.
+   - If you don't know it, run the app without it, and it will list all your zones with their name and ID.
+
+4. Run the script!
+
+```
+pnpm run start
+```
+
+[cf-token]: https://dash.cloudflare.com/profile/api-tokens
+[pnpm]: https://pnpm.io/installation#using-a-standalone-script