Verify and flesh out list of primitives

[nicolaspayette]

@cbradyatinquire, I have added a list of primitives with brief descriptions to the README. Those descriptions are, however, very limited and mostly guesswork since I know next to nothing about Arduinos and have no way to test your extension. Would you mind checking that I didn't make any blunders and, where appropriate, flesh out the descriptions a bit?

[nicolaspayette]

Note that the content of the README will be used to generate the extension's documentation for http://ccl.northwestern.edu/netlogo/docs/.