Update readme to make it clear AMP plugin is now supporting both Website and Stories experiences

[westonruter]

The AMP plugin is no longer about just the Website: it also includes the Stories experience. The readme needs to be rewritten to make it clear that you can use the AMP plugin either to make an AMP Website or to just publish AMP Stories on your site, or both.

[westonruter]

In v1.3 we should enable the Stories experience by default for new installs, as par #2550 (comment):

Good question. Not sure. Part of me says no because historically it would be adding something that people didn't ask for. Jetpack's custom content types are disabled by default.

I'd say leave it off by default now, and then we need to revamp the readme.txt to indicate that Stories is a key feature alongside Websites. (That's something that should be done prior to the 1.2 release.)

Then in 1.3 we can make it enabled by default, especially when Gutenberg is no longer a dependency, and when the Stories editor is more mature.

This would be doing:

--- a/includes/options/class-amp-options-manager.php
+++ b/includes/options/class-amp-options-manager.php
@@ -35,7 +35,7 @@ class AMP_Options_Manager {
 	 * @var array
 	 */
 	protected static $defaults = array(
-		'experiences'              => array( self::WEBSITE_EXPERIENCE ),
+		'experiences'              => array( self::WEBSITE_EXPERIENCE, self::STORIES_EXPERIENCE ),
 		'theme_support'            => 'disabled',
 		'supported_post_types'     => array( 'post' ),
 		'analytics'                => array(),

Again, this would only apply to new installs. Existing installs that already have the experiences option set, would not get the stories experience added to their site. But for new installs that have installed the plugin knowing that it features AMP for websites and stories, then they'll get both by default upon activation.

[westonruter]

Done in #2571