update README with up-to-date information and direct link to download

[aparrish]

The Processing library reference documentation links directly to this repository, but the repository itself has no information about how to actually install the library! I think this fact is leading to questions like these. Even aside from this fact, the README is out-of-date and has misleading information (e.g., that Windows isn't supported) that might potentially send users on wild goose chases.

The README file should be updated with a clearer description of what is in this repository, a description of how to add the library inside the Processing IDE (or a link to a tutorial about how to use the "Add Library..." feature).

Also, I'm working on a tutorial for using Processing.py on the command line, where (obviously) the "Add Library..." feature isn't available. It would be super helpful if the README also had a direct link to the file that users need to download to use the library with their sketches (when not using the IDE's "Add Library..." feature). It's easy enough to find this file under "Releases" but I don't think that location is obvious to people unfamiliar with Github.

Thank you so much for your hard work on this library! Please excuse the bug report—I realize this should be a pull request, but the proposed changes to the README are extensive enough as to go beyond my expertise (I wouldn't in any way want to misrepresent the library, how it works or how to build it).

[wirsing]

@aparrish thanks for all the updates / hints! I'll merge that into the helpfiles and README for the next release.