Reorganize and touch-up Adding to Phoenix section

[nskins]

Hi! I just went through the process of installing tailwind in my Phoenix application, and I have a few changes to the README that might help future readers.

Summary of changes:

• Moved section on updating assets.deploy to be after adding the tailwind dependency in mix.exs. This streamlines the necessary changes so that the reader does not have to come back to mix.exs later.
• Instructed the user to run mix tailwind.install at the end. I explained what effects this command has on the files in the project.
• Removed instructions to check assets/js/app.js. mix tailwind.install removes the line in question for you, so there's no need to check.
• Removed "four" from the number of steps. I think it's not necessary to list the number of steps since things can change and that number might not be updated. It might be five steps anyway if you include the one for umbrella projects.

[josevalim]

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