Write down existing excluded categories from twofactorauth.org

EXCLUSION.md

@@ -1,3 +1,25 @@
-TwoFactorAuth.org Blacklist
-=================
+# TwoFactorAuth.org excluded categories and websites
 
+Below is a list of categories and websites that we've opted out from listing on TwoFactorAuth.org. If you want to make a copy (fork) of our site to list any of these sites you're welcome to do so as long as you comply with our [license](https://github.com/2factorauth/twofactorauth/blob/master/LICENSE).
+
+## Categories
+
+*   #### Pornographic sites
+
+    As the volunteers of [2factorauth](https://github.com/2factorauth) need to go through all sites to validate that the site/service provides 2FA to it's users and verify that the pull request meets our style guidelines we've chosen not to list pornographic sites.  
+    We've also chosen to do this so that we don't have to go through potentially disturbing or illegal material.
+
+*   #### Self-hosted services
+
+    The category "self-hosted services/sites" includes all types of software that's designed to be hosted by the end user (such as [ownCloud](https://owncloud.org/), [NextCloud](https://nextcloud.com/), [Mastodon](https://joinmastodon.org/), [phpBB](https://www.phpbb.com/), [XenForo](https://xenforo.com/) etc).  
+    It also includes [Wordpress.org](https://wordpress.org/) (the open source software) but not [Wordpress.com](https://wordpress.com/) (the company).
+
+    There are several resons for why we've opted to not list such sites and services.
+
+    -   TwoFactorAuth.org is targeted towards consumers and not website administrators. As of right now, very few general internet users choose to host their own websites and services. Therefore we think that the effort on our part to maintain such a list of self hosted alternatives would outweigh the minimal theoretical change in the internet landscape by listing such services.
+
+    -   If the core project doesn't support two factor authentication but instead through plugins it would mean that we would have to list all plugins that enable two factor authentication for the service. Something that is currently impossible with out website layout.
+
+## Sites
+
+Below is a list of specific sites/services excluded from TwoFactorAuth.org.