Clarify instructions

[LeaVerou]

• Made it clear that Eleventy needs to be upgraded before running this plugin. That was not clear to me at first, I thought the plugin itself would upgrade it, or it would tell me if it's safe to upgrade.
• Added instructions on how to get output from this plugin (running the usual build command), and clarified when exactly to remove it
• Removing the plugin from package.json doesn't remove it from node_modules, whereas npm uninstall does.

[SHiLLySiT]

I went looking thru issues/PRs specifically for the second bullet point as I assumed there was going to be a second command to run, so this was very helpful!

Also, if anyone is using Github Actions to deploy sites, you may need to bump your Node version in your Github Actions main.yml as eleventy now requires Node 14+.

[zachleat]

Thank you for this! I may tweak it a bit as we discourage folks to globally install eleventy now but it’s great! https://www.11ty.dev/docs/global-installation/

[LeaVerou]

I may tweak it a bit as we discourage folks to globally install eleventy now but it’s great! 11ty.dev/docs/global-installation

Great! Global installation is a huge footgun, glad to see it's discouraged now.