Add the 'Conditionals' lesson to csintro. (#428)

docs/courses/csintro/conditionals.md

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+# Conditional statements
+
+This lesson introduces the Logic blocks such as 'If...then' and 'If...then...else'.
+Students practice skills of creativity, problem-solving, and collaboration.
+
+## Lesson objectives
+Students will...
+
+* Understand what conditional statements are, and why and when to use them in a program.
+* Learn how to use the Logic blocks 'If...then' and 'If…then...else'.
+* Practice using the Logic blocks so different conditions yield specified outcomes.
+* Demonstrate understanding and apply skill by collaborating with classmates to create a game that uses a micro:bit and a program that correctly and effectively uses conditionals. 
+
+## Lesson structure
+* Introduction: Conditionals in daily life
+* Unplugged Activity: Red if, Green then
+* Micro:bit Activity: Rock Paper Scissors
+* Project: Board Game
+* Assessment: Rubric for board game project
+* Standards: Listed
+
+## Lesson plan
+
+1. [**Overview**: Conditional statements](/courses/csintro/conditionals/overview)
+2. [**Unplugged**: Red light, green light](/courses/csintro/conditionals/unplugged)
+3. [**Activity**: Rock, paper, scissors](/courses/csintro/conditionals/activity)
+4. [**Project**: Board game](/courses/csintro/conditionals/project)
+
+## Related standards
+
+[Targeted CSTA standards](/courses/csintro/conditionals/standards)