Update README and .gitignore for .vimrc.local

Signed-off-by: Tim Ekl <lithium3141@gmail.com>
Set Git to use push.default = upstream

Recent versions of Git complain if push.default isn't set, but the new
default ("simple") will also complain if local and remote branch names
aren't the same. Since I have that setup in some places, use "upstream"
pushing by default, which only pushes the current branch to the remote,
but disregards name differences.

Signed-off-by: Tim Ekl <lithium3141@gmail.com>