Update plan.md to include source-method configuration

- Add source-method: pre-packaged-source to composer.json example
- Document that this field is required for PIE to use pre-packaged source
- Add link to MongoDB extension implementation

Author: oschwald

plan.md

@@ -7,14 +7,15 @@ Set up a new extension repository (`MaxMind-DB-Reader-php-ext`) that uses this r
 
 PIE downloads source code as ZIP archives from Composer/GitHub, which don't include git submodule contents. To work around this limitation, we use PIE's built-in support for **pre-packaged source archives**:
 
+- Set `"source-method": "pre-packaged-source"` in the `php-ext` section of composer.json
 - The release script creates a tarball: `php_maxminddb-{version}-src.tgz`
 - This tarball includes the full ext/ directory from the submodule
 - It's uploaded as a GitHub release asset
 - PIE automatically detects and uses this pre-packaged source instead of the ZIP archive
 
 This is the same approach used by the MongoDB extension and others with submodule dependencies. See:
 - https://github.com/php/pie/issues/39
-- MongoDB extension implementation
+- MongoDB extension implementation: https://github.com/mongodb/mongo-php-driver
 
 ## Phase 1: Create Extension Repository Structure
 
@@ -50,6 +51,7 @@ Initial files needed:
     "priority": 50,
     "support-zts": true,
     "support-nts": true,
+    "source-method": "pre-packaged-source",
     "build-path": "MaxMind-DB-Reader-php/ext",
     "configure-options": [
       {