Addition for README file

[antofthy]

What happened:

I am just trying out dive for the first time, with no introduction other that the github page.
My windows are by default 80 columns wide (I prefer them like that)
When dive is run in 80 columns, the left (layer) pane is tiny and rather useless..
I almost gave up on using dive as I could not find anything on enlarging the left pane!
BY ACCIDENT I later ran it in a window with a larger number of columns, and suddenly the left pane became useful

What you expected to happen:

Something in the README along the lines...

Be sure to run in a terminal with at least 120 columns

Yes I know obvious to all you people who use 'screen', but I prefer separate terminal windows.

How to reproduce it (as minimally and precisely as possible):

Run it in a standard 80 column window! Number of lines does not matter.

Anything else we need to know?:

Built in help on switches and view modes would be good. Say starting with a popup when you press '?'

Environment:

• OS version
  redhat linux
• Docker version (if applicable)
  dive 0.11.0