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Update Installation setcion

And use hash marks instead of underscores for using various header tags.
Delete trailing spaces
Fix documentation about init files

Because it is better to use `~/.emacs.d/init.el` than `~/.emacs`.
Merge pull request #19 from kyagi/update-document

Yoshida-san, thank you for this heads-up. :flashlight: :satisfied:
Add pop-in and pop-out hooks
Merge pull request #20 from kyagi/add-hook

Thank you, Yoshida-san. :)
Update document about hooks
Add shell-pop-full-span to get a terminal the width of which is the same as that of the Emacs window
Merge pull request #21 from popoppo/full-span

Add shell-pop-full-span to get a wide terminal
Update README.md
Update README.md