Update "How do I run it" in README

[Fuco1]

Elsa is now on MELPA so that should be the default channel.

• update the Cask instructions (should still be default)
• expand the flycheck section (how to install, what to setup in Emacs)
• mention emake.el (@vermiculus can you write a short paragraph on how to run it?)
• mention how to run it "DIY style" (not recommended)

[vermiculus]

Here's what I would say for EMake:

Install EMake via the usual means (bash <(curl -fsSL https://raw.githubusercontent.com/vermiculus/emake.el/master/new)). You can now run Elsa's checks with make lint-elsa.

I say 'usual' like it's been around for a while – but I only wrote it this evening being prompted by this issue 😄 I realized there were too many steps for install (previously: well, everything in that script).

[Fuco1]

@vermiculus thanks, that is simpler than I expected :D

[vermiculus]

😉 Note actual Elsa support is dependent on vermiculus/emake.el#27.