added CONFIGURATION.md containing some details about configuring mrf

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+# Configuring Pleroma
+
+In the `config/` directory, you will find the following relevant files:
+* `config.exs`: default base configuration
+* `dev.exs`: default additional configuration for `MIX_ENV=dev`
+* `prod.exs`: default additional configuration for `MIX_ENV=prod`
+
+Additionally, you can overload options in the following files:
+* `dev.secret.exs`: additional custom configuration for `MIX_ENV=dev`
+* `prod.secret.exs`: additional custom configuration for `MIX_ENV=prod`
+
+## Message Rewrite Filters (MRFs)
+
+Modify incoming and outgoing posts.
+
+    config :pleroma, :instance,
+      rewrite_policy: Pleroma.Web.ActivityPub.MRF.NoOpPolicy
+
+`rewrite_policy` specifies which MRF policies to apply. It can either be a single policy or a list of policies.
+Currently, MRFs availible by default are:
+* `Pleroma.Web.ActivityPub.MRF.NoOpPolicy`
+* `Pleroma.Web.ActivityPub.MRF.DropPolicy`
+* `Pleroma.Web.ActivityPub.MRF.SimplePolicy`
+* `Pleroma.Web.ActivityPub.MRF.RejectNonPublic`
+
+Some policies, such as SimplePolicy and RejectNonPublic, can be additionally configured in their respective sections.
+
+### NoOpPolicy
+
+Does not modify posts (this is the default `rewrite_policy`)
+
+### DropPolicy
+
+Drops all posts. It generally does not make sense to use this in production.
+
+### SimplePolicy
+
+Versatile policy for applying effects to posts from certain instances.
+
+    config :pleroma, :mrf_simple,
+      media_removal: [],
+      media_nsfw: [],
+      federated_timeline_removal: [],
+      reject: []
+
+* `media_removal`: posts from these instances will have attachments removed
+* `media_nsfw`: posts from these instances will have attachments marked as nsfw
+* `federated_timeline_removal`: posts from these instances will be marked as unlisted
+* `reject`: posts from these instances will be dropped
+
+### RejectNonPublic
+
+Drops posts with non-public visibility settings.
+
+    config :pleroma :mrf_rejectnonpublic
+      allow_followersonly: false,
+      allow_direct: false,
+
+* `allow_followersonly`: whether to allow follower-only posts through the filter
+* `allow_direct`: whether to allow direct messages through the filter