mod_mix: Add readme

mod_mix/README.markdown

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+This module enables prosody to use Mediated Information Exchange with
+the following protocols being supported:
+
+- MIX-CORE
+- MIX-ADMIN
+
+Note that this module does not provice MIX-PAM, which is need for MIX clients to function.
+
+| Option | Description | Default |
+| --- | --- | --- |
+| `default_description` | The default description of a newly created channel | `""` |
+| `default_name` | The default name of a newly created channel | `"MIX Channel"` |
+| `service_name` | The name of the MIX service during service discovery | `"Prosody MIX Service"` |
+| `restrict_channel_creation` | Controls who can create channels | `"local"` |
+
+`restrict_channel_creation` has two modes of operation:
+
+- `"local"`: Only local users can create channels, e.g. `romeo@example.org` can create channels on `mix.example.org`, but not `juliet@example.net`
+- `{ ... }`: If the value is a table, then one can list hosts or users that are allowed to create channels
+
+## Example configuration
+```lua
+Component "mix.example.org" "mix"
+    service_name = "My name Jeff"
+    restrict_channel_creation = { "exmaple_org" }
+```