misc/vim/readme.txt: workaround weird OS X vim bug.

misc/vim/readme.txt: workaround weird OS X vim bug.
The vi bundled with OS X has a weird bug in that if you turn off
filetype in .vimrc when it's not turned on, even a clean exit of
vi will return 1 which breaks almost everything.

While we're at it, add hint to change $GOROOT to its actual value
in .vimrc.

Fixes #7865.

LGTM=robert.hencke, dsymonds
R=golang-codereviews, robert.hencke, gobot, dsymonds
CC=golang-codereviews
https://golang.org/cl/96800046