Document files/directories requiring backmerging to WP Core for major…

… release (#58064)

* Initial docs stub

* Remove phpunit from list

* Flatten ignore list for absolute clarity

* Correct title

docs/contributors/code/back-merging-to-wp-core.md

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+# Back-merging code to WordPress Core
+
+For major releases of the WordPress software, Gutenberg features need to be merged into WordPress Core. Typically this involves taking changes made in `.php` files within the Gutenberg repository and making the equivalent updates in the WP Core codebase.
+
+## Files/Directories
+
+Changes to files within the following files/directories will typically require back-merging to WP Core:
+
+-   `lib/`
+-   `phpunit/`
+
+## Ignored directories/files
+
+The following directories/files do _not_ require back-merging to WP Core:
+
+-   `lib/load.php` - Plugin specific code.
+-   `lib/experiments-page.php` - experiments are Plugin specific.
+-   `packages/block-library` - this is handled automatically during the packages sync process.
+-   `packages/e2e-tests/plugins` - PHP files related to e2e tests only. Mostly fixture data generators.
+-   `phpunit/blocks` - the code is maintained in Gutenberg so the test should be as well.
+
+Please note this list is not exhaustive.
+
+## Pull Request Criteria
+
+In general, all PHP code committed to the Gutenberg repository since the date of the final Gutenberg release that was included in [the _last_ stable WP Core release](https://developer.wordpress.org/block-editor/contributors/versions-in-wordpress/) should be considered for back merging to WP Core.
+
+There are however certain exceptions to that rule. PRs with the following criteria do _not_ require back-merging to WP Core:
+
+-   Does not contain changes to PHP code.
+-   Has label `Backport from WordPress Core` - this code is already in WP Core.