Create introduction.md

Author: quintuple-mallard

exercises/concept/captains-log/docs/introduction.md

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+# Introduction
+
+Many programs need (apparently) random values to simulate real-world events.
+
+Common, familiar examples include:
+
+  - A coin toss: a random value from ('H', 'T').
+  - The roll of a die: a random integer from 1 to 6.
+  - Shuffling a deck of cards: a random ordering of a card list.
+  - The creation of trees and bushes in a 3-D graphics simulation.
+
+Generating truly random values with a computer is a [surprisingly difficult technical challenge][why-randomness-is-hard], so you may see these results referred to as "pseudorandom".
+## Generating random numbers
+In Javascript, you can generate psuedorandom numbers using the [`Math.random()`][Math.random] function.
+It will return a psuedorandom floating-point number between 0 (inclusive), and 1 (exclusive).
+
+To get a random number between _min_ (inclusive) and _max_ (exclusive) you can use a function something like this: 
+```javascript
+function getRandomInRange(min, max) {
+  return min + Math.random() * (max - min) ;
+}
+getRandomInRange(4, 10)
+// => 5.72
+```
+## Generating random integers
+To generate a random integer, you can use `Math.floor()` or `Math.ceil()` to turn a randomly generated number into an integer.
+~~~~exercism/caution
+
+The `Math.random()` function should NOT be used for security and cryptographic applications!!
+
+Instead, you can use the Web Crypto API, which provides various cryptographic functions.
+~~~~
+
+[why-randomness-is-hard]: https://www.malwarebytes.com/blog/news/2013/09/in-computers-are-random-numbers-really-random
+[Math.random]: https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Math/random