extended docs to show conditional shortcut execution

[janpaepke]

I had other extensions using the same shortcut I wanted to use for duplication, so I had to find out how to make it conditional so that it would only run, when I focus the file in the explorer.

Thought this might be helpful information for other users as well.

What is the purpose of this pull request?

Improve readme.

What changes did you make? (Give an overview)

Added some json to readme.

[janpaepke]

*bump

[ghost]

Here's mine.

    {
        "key": "ctrl+d",
        "command": "duplicate.execute",
        "when": "explorerViewletVisible && filesExplorerFocus && !explorerResourceIsRoot && !inputFocus"
    },