Add information about SelectableTextBlock

[SKProCH]

There is issue in Material.Avalonia about TextBlock in which user can select text: https://github.com/AvaloniaCommunity/Material.Avalonia/issues/278

Seems like because our control named SelectableTextBlock, there is a quite small chances to find it somehow(because the WPF/UWP analogue is called RichTextBox). I think that we need a separate page for the awesome control.