OpenCilk programmer's guide

Here is a place where we can add a concise reference guide about how to write a correct program with OpenCilk. (This would be very different than the "language reference" -- which gives "the final word" to "the language police" about what OpenCilk is.)

We might model the reference guide after one or more of these examples:

• GNU C reference manual, for example here – for C eagles.
• Julia manual, for example here – for Julia eagles.
• Julia developer’s documentation, for example here – for Julia owls.
• Numpy reference, for example here – for Numpy eagles.