Add the 'Variables' lesson to csintro. (#427)

docs/courses/csintro/variables/overview.md

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+# Introduction
+
+Computer programs process information.  Some of the information that is input, stored, and used in a computer program has a value that is **constant**, meaning it does not change throughout the course of the program.  An example of a **constant** in math is ‘pi’ because ‘pi’ has one value that never changes.  Other pieces of information have values that **vary** or change during the running of a program.  Programmers create **variables** to hold the value of information that may change.  In a game program, a variable may be created to hold the player’s current score, since that value would change (hopefully!) during the course of the game.
+
+Ask the students to think of some pieces of information in their daily life that are **constants** and others that are **variables**.
+
+* What pieces of information have values that don’t change during the course of a single day (constants)? 
+* What pieces of information have values that do change during the course of a single day (variables)
+Constants and variables can be numbers and/or text.
+
+## Examples 
+In one school day...
+
+* Constants: The day of the week, the year, student’s name, the school’s address
+* Variables: The temperature/weather, the current time, the current class, whether they are standing or sitting...
+
+Variables hold a specific type of information. The micro:bit's variables can keep track of numbers, strings, booleans, and sprites. The first time you use a variable, its type is assigned to match whatever it is holding. From that point forward, you can only change the value of that variable to another value of that same type.
+
+* A number variable could hold numerical data such as the year, the temperature, or the degree of acceleration.
+* A string variable holds a string of alphanumeric characters such as a person's name, a password, or the day of the week.
+* A boolean variable has only two values: true or false. You might have certain things that happen only when the variable called _gameOver_ is false, for example.
+* A sprite is a special variable that represents a single dot on the screen and holds two separate values for the row an