Update README: Add options(vsc.helpPanel = ...)

[kar9222]

What problem did you solve?
Update README.md for options(vsc.helpPanel = "Two" | "Active" | "Beside" | FALSE)

(If you have)Screenshot
No

(If you do not have screenshot) How can I check this pull request?
Run the codes in terminal and check how the help panel opens up e.g. current editor or 2nd editor group

# Default
getOption('vsc.helpPanel')
# NULL
?mean
# Help panel opens in the 2nd editor group 

options(vsc.helpPanel = 'Active')
?mean
# Help panel opens in current editor

options(vsc.helpPanel = 'Two')
?mean
# Help panel opens in current editor

options(vsc.helpPanel = 'Beside')
?mean
# Help panel shows besides the current editor

options(vsc.helpPanel = FALSE)
?mean
# Disable help panel and revert to old behaviour 

[kar9222]

Didn't know about this when I edited the README.md. This was done automatically by VSCode with my settings.json editor.trimAutoWhitespace: true