add processing2hpgl library (#57)

* added new processing library

* update link and language a little

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Co-authored-by: Brian Boucheron <brian@boucheron.org>

README.md

@@ -79,6 +79,7 @@ HPGL is a serial/text-based protocol used by most old pen plotters, and even man
 - [HPGL Viewer](https://github.com/drskullster/HPGLViewer) - An HPGL Viewer using JavaScript and HTML5 canvas.
 - [HPGL Sender](https://github.com/LgHS/hpgl-sender) - A web interface for previewing HPGL and sending it to your plotter.
 - [HPGLGraphics](https://github.com/ciaron/HPGLGraphics) - A Processing library for writing HPGL files.
+- [processing2hpgl](https://github.com/awdriggs/processing2hpgl) - A Processing library that allows for direct communication with HPGL pen plotters from within a Processing sketch.
 
 ### G-code