polish Install section of README.md for clarity

[rickhull]

Minor polish items to minimize awkwardness and maximize clarity

• add backticks for monospace code/command highlighting
• 80ch line length
• emacs initialization file rather than assume .emacs

[tonini]

Thanks @rickhull for your contribution. <3

But I thought already about the Installation part in the README and thinking more about something like this: https://github.com/tonini/alchemist.el#installation

It a kind of installation guidance build from cider and takes the best informations needed together.

Would you like to update it more in that direction?

[rickhull]

@tonini Yeah, that seems sensible. But I'm probably not the best person person to make sweeping changes. I'm just a very new user of elixir-mode who had trouble with the install process. That said, I am always interested in better, clearer documentation, and I can dive right in to making small tweaks and edits.

How about we merge this one for now, and I'll take a stab on the revamp in a subsequent PR?