Discuss concurrency vs parallelism, taking inspiration from javascript (#176)

Author: glennj

exercises/practice/parallel-letter-frequency/.docs/instructions.append.md

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-You can read about concurrency in Wren [in the manual][concurrency].
+## Concurency vs. Parallelism
+
+Wren provides _concurrency_ via a construct called _fibers_.
+From the Wren manual:
+
+> Fibers are a bit like threads except they are cooperatively scheduled.
+> That means Wren doesn’t pause one fiber and switch to another until you tell it to.
+
+Wren's execution is single-threaded so we don't actually get _parallelism_.
+
+Here's a quick definition for each that illustrates the diferences between the two:
+
+- Concurrency is when two or more tasks can start, run and complete in overlapping time periods, being executed by the same processing unit.
+- Parallelism is when two or more tasks can start and run at the same time, being executed independently of eachother by separate processing units.
+
+Read more about [concurrency in Wren][concurrency].
 
 [concurrency]: https://wren.io/concurrency.html