Add link to other instances list

This should help federation

app/views/about/index.html.haml

@@ -32,6 +32,7 @@
       = link_to t('about.learn_more'), about_more_path
       = link_to t('about.terms'), terms_path
       = link_to t('about.source_code'), 'https://github.com/tootsuite/mastodon'
+      = link_to t('about.other_instances'), 'https://github.com/tootsuite/mastodon/blob/master/docs/Using-Mastodon/List-of-Mastodon-instances.md'
 
     = link_to t('about.get_started'), new_user_registration_path, class: 'button webapp-btn'
     = link_to t('auth.login'), new_user_session_path, class: 'button webapp-btn'