Moved manifest file to the root directory of the plugin

[kvijaykiran]

Instructions to pull a plugin directly from a git URL say that the file manifest.json should be in the root directory instead of 'Packages'. I moved the file to the root directory. I do not know if this will work, but I wanted to give this a try.

[mlavik1]

@kvijaykiran I never tried pulling directly from this repo through Unity, but did you figure out what would be the right approach?
It's something I'd like to support, but I'd also like to allow people to add this repo directy under "Assets". I know some do that, though git submodules, which can be practical if they're contributing to the plugin as well.