Updated readme file.

Added detailed description with screen shots and links.
Find file extensions: Add support for multiple patterns matching.
Find files: Add signals for starting and stopping a search.
Find file extension: Support for saving previously used patterns.
Main window: Add default option for second level extension.
Preferences: Add option for second level extension.
Plugin base: Item list: Preload second level extension option.
Plugins: File list: Support split file name with seconf level extension.
Emblem manager: Added missing default emblems.
Widgets: Emblem renderer: Add support for links.
Plugin base: Provider: Add follow param to get_stat method.
Plugins: File list: Update local provider to support get_stat follow.
Plugins: File list: Update Gio provider to support get_stat follow.
Plugins: File list: Properly handle symbolic links.