Reorder and refresh readme for 2021

[tabacitu]

Hey @mewebstudio ,

I took the liberty of polishing the README a little bit, but these changes are opinionated so if you don't like them... just close the PR, no problem 😉

Here's what I did:

• HEADER
  • provided a link to the original HTMLPurifier, so people can click to go there
  • provided a description of HTML Purifier, for SEO and so that noobs more easily understand what this does
• INSTALLATION
  • added this section (it was missing, and the installation info was under "Usage")
  • reordered the installation instructions from the newest Laravel version to the oldest
• USAGE
  • without the installation instructions, what's remaining in this section is cleaner now, I think
• CONFIGURATION
  • moved this section after usage (since most people probably won't need it)
• FOOTER
  • Added quick sections Changelog, Security, Credits and License to match industry best practices

I think the end result looks more professional, explains a lot quicker what this package is about. Helps most people faster (most people will be using new Laravel versions nowadays) but still provides all the information that was before. It's mostly reordering, not rewording.

Again: don't like it, don't merge it, no need to explain your reasoning.
I personally think this is better but you might not 😄

Cheers!

[tabacitu]

You can preview how the refreshed README looks here - https://github.com/tabacitu/Purifier/tree/patch-1