docs: Add steps for uploading to ppa (#17)

README.md

@@ -308,6 +308,21 @@ location /d/ {
 }
 ```
 
+## For developers
+
+### Upload to the `ppa` repository
+
+1. Follow the [PGroonga documentation](https://pgroonga.github.io/development/release.html#requirements) and do the required setup
+   * The environment variables used in this repository are `GROONGA_REPOSITORY`, `LAUNCHPAD_UPLOADER_PGP_KEY` and `APACHE_ARROW_REPOSITORY`
+2. Install `nginx-dev`
+   * Install `nginx-dev` as it is also required
+   * Note: The working machine must be Ubuntu 24.04+ or for Debian bookworm+
+3. Upload
+   * Run in the repository root directory
+     ```bash
+     rake package:ubuntu
+     ```
+
 ## License
 
 LGPLv2.1 or later.