Add licence, adjust project layout & create Unity package

[nitz]

Hello!

I've had some variation of a serialized dictionary in basically every project I've done, so seeing yours with a nice feature set looks like it will definitely save time in the future.

I try to stick with using packages through Unity as often as possible, since they handle versioning much better and can pull directly from git! That's why I wanted to offer these changes so that you or others could use this in the same way.

My proposed changes here are a few, and I made a few assumptions:

• I've added a README.md. In it, I've just duplicated the documentation you wrote in the PDF. I had to break the images out so they could be put inline in the markdown.
• I moved the Sample folder to Samples~/Usage. Unity's package recommendations include placing samples in the Samples~ folder, so they don't get tracked & imported with the rest of the package by default. There's also a mechanism that allows the samples to be imported automatically by the user, via the package manager interface, like so.
• Lastly, I created the package.json. This is the file that the Unity Package Manager uses to populate information about the package in the interface.

Part of the unity package data is the license the package is released under. Your repository didn't have a license at all, so I added the MIT license, since I noticed you preferred it for other repositories. If you'd rather use something different, feel free to amend that commit or let me know what you'd like and I can do it!

Cheers, and thanks for sharing!

[ayellowpaper]

Amazing work, thanks. I thought about converting this into a package, but didn't get around to it.