add readme for the calendar widget

calendar-widget/README.md

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+# Calendar Widget
+
+Calendar widget for Awesome WM - slightly improved version of the `wibox.widget.calendar`. Also supports mouse scroll up/down in order ot switch month - scroll up - shows next month, scroll down - previous.
+
+Top center placement:
+
+![calendar_top](./calendar_top.png)
+
+Top right placement:
+
+![calendar_top_right](./calendar_top_right.png)
+
+The placement is setup in theme.lua by `calendar_placement` variable, currently supported `top` (default), `top_right`, `bottom_right`. 
+
+# How to use
+
+This widget need an 'anchor' - another widget which triggers visibility of the calendar. Default `mytextclock` is the perfect candidate!
+
+```lua
+local calendar_widget = require("awesome-wm-widgets.calendar-widget.calendar")
+-- ...
+-- Create a textclock widget
+mytextclock = wibox.widget.textclock()
+mytextclock:connect_signal("button::press",
+    function(_, _, _, button)
+        if button == 1 then calendar_widget.toggle() end
+    end)
+```