Add XEP-0333 Docs

[theowenyoung]

This PR addresses #

Proposed changes include:

• describe the functionality changes
• describe new or updated tests
• describe changes to the documentation

By

MongooseIM/src/mongoose_chat_markers.erl

Lines 1 to 20 in 9690e60

	%%%-------------------------------------------------------------------
	%%% @copyright (C) 2021, Erlang-Solutions
	%%% @doc
	%%% This module implements [XEP-0333: Chat Markers] helper functions
	%%% @end
	%%%-------------------------------------------------------------------
	-module(mongoose_chat_markers).
	-include("mongoose_ns.hrl").
	-include_lib("exml/include/exml.hrl").
	%% Markers
	-export([has_chat_markers/1,
	list_chat_markers/1,
	chat_marker_names/0]).
	-type id() :: binary().
	-type chat_marker_type() :: received | displayed | acknowledged.
	-type chat_marker_element() :: {chat_marker_type(), id()}.
	-export_type([chat_marker_type/0]).

Does it seem the chat markers feature can list in the doc?

[chrzaszcz]

Thanks for the PR, looks good