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

docs/courses/csintro/variables.md

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+# Variables
+
+This lesson introduces the use of variables to store data or the results of mathematical operations.  Students will practice giving variables unique and meaningful names.  And we will introduce the basic mathematical operations for adding subtracting, multiplying, and dividing variables.
+
+![Variable value](/static/courses/csintro/variables/variable-value.png)
+
+## Lesson Objectives 
+
+Students will...
+
+* Understand what variables are and why and when to use them in a program.
+* Learn how to create a variable, set the variable to an initial value, and change the value of the variable within a micro:bit program.
+* Learn how to create meaningful and understandable variable names.
+* Understand that a variable holds one value at a time.
+* Understand that when you update or change the value held by a variable, the new value replaces the previous value.
+* Learn how to use the basic mathematical blocks for adding, subtracting, multiplying, and dividing variables.
+* Apply the above knowledge and skills to create a unique program that uses variables as an integral part of the program.
+	
+## Lesson Structure
+
+* Introduction: Variables in daily life
+* Unplugged Activity: Rock Paper Scissors scorekeeping activity
+* Micro:bit Activity: Make a game scorekeeper
+* Project: Make a scorekeeper
+* Project Mods
+* Assessment: Rubric 
+* Standards: Listed
+
+## Lesson plan
+
+1. [**Overview**: Variables and constants](/courses/csintro/variables/overview)
+2. [**Unplugged**: Keeping score](/courses/csintro/variables/unplugged)
+3. [**Activity**: Scorekeeper](/courses/csintro/variables/activity)
+4. [**Project**: Everyting counts](/courses/csintro/variables/project)
+
+## Related standards
+
+[Targeted CSTA standards](/courses/csintro/variables/standards)