Add and release OpenUPM support.

[f1yingbanana]

This PR:

• adds OpenUPM support by copying the content of the UPM package in the release process back to the repo,
• updates the README to reflect latest steps in using EDM4U, and
• generates artifacts for a new release containing these changes

[a-maurice]

In general, I'm guessing you used a markdown editor that autoformatted this file a bunch. The problem is the formatting doesn't seem to match what GitHub expects, so it breaks things like the bullet points, and it also makes it hard to tell what was actually changed. Any chance you can limit this just to the significant changes?

[f1yingbanana]

Yeah, I ran this with hg fix, but I believe the formatting results are displayed correctly. Please take a look at the preview on my fork.

I agree that I might have made more changes than I originally intended when I went through the entire README file. Here are the list of changes I've made, let me know which ones you want me to revert:

1. Adding OpenUPM badges to the top of the page.
2. Formatting headings, so there is only one h1 (title). Essentially this moves all headings down one level.
3. Adding a section on who should do what with EDM4U (target audience) to Overview.
4. Rewriting Getting Started section to include exactly how to download and install EDM4U, with a section on conflict resolution.
5. Moving the section on embedding EDM4U in other packages (originally in Getting Started) into its own section below, since it is no longer necessary nor endorsed.
6. Unifying all component usages under a Usages section.
7. Refactoring the repetitive download sample config file bullet list in each of the component usage sections into Getting Started section.
8. Moving Requirements, Getting Started, and Usages above Background, in line with general README guides (see Guide to writing on Readme.md (.markdown)file for GitHub project, How to Write a Good README File for Your GitHub Project. The motivation is that Background is good information, but is less important than how to install and use EDM4U.
9. Clarifying build requirement (Unity 2021 or lower, must build with Java 11).
10. Running hg fix on the markdown file.