Update docs with more information on powerline colors
Update docs with more information on powerline colors

The powerline color issue is due to the fact that Emacs 24.4 now
defaults to using sRGB colorspace on OS X 10.7 and higher, not some
"brokeness" with brew's `emacs` build as the previous verbiage seemed to
imply; the powerline codebase is still assuming `(setq
ns-use-srgb-colorspace nil)` is the default.

This PR adds more information about this so that users may
decide whether they want to just set it back to nil, switch to
emacs-mac (which has it set to nil anyway), or disable powershell
separators. (Turning off srgb colorspace results in washed-out theme
colors).

--------

Side rant:
I don't agree that the recommended fix for this would be to use
`emacs-mac-port` as that basically just gives better trackpad support.
IMHO trackpad support does not outweigh the frustrations over default
meta/super key bindings, no `server-start` support, or the use of the
antiquated carbon api, but I left the recommendation as-is in this PR.

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