add pointer to user docs

[xulman]

In the sw itself, provide an item that would open a web browser that would open the README.md

• Introduce a button "Doc" or "Help" or alike into the Controls panel into the lowest row between the "Repaint" and "Close"
• Place this button also into the "Mastodon sciview Bridge Keys Overview" panel which pops up when user clicks sciview -> Help -> Mastodon Bridge