Add WordPress.org distribution file

Specify which files should not be included in the push to WordPress.org. These are all development files and directories.

Note that `/vendor` is not included here - that means that it WILL be available in the plugin on WordPress.org, which is essential as we have some `require` dependencies.

The deploy Action will use rsync + `.distignore` if the `.distignore` exists, so it doesn't care what may or may not be ignored via `.gitignore`.

See https://plugins.trac.wordpress.org/browser/fb-instant-articles#trunk

.distignore

@@ -0,0 +1,14 @@
+/.distignore
+/.editorconfig
+/.git
+/.gitignore
+/.github
+/.phpcs.xml.dist
+/.phpunit.result.cache
+/.wordpress-org
+/bin
+/composer.json
+/phpunit.xml.dist
+/release.sh
+/tests
+