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

docs/courses/csintro/conditionals/activity.md

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+# Activity: Rock, paper, scissors
+
+For this activity, each student will need a micro:bit. 
+Everyone will create the same program, the classic rock paper scissor game.
+
+![Rock, paper, scissors](/static/courses/csintro/conditionals/rock-paper-scissors.jpg)
+
+## Introduce activity
+* Have students recall the classic rock paper scissors game.
+* What are the rules of the game? What are the conditionals?
+>Example: If Player A gets rock, and Player B gets scissors, Then Player A wins.
+* Have students write the pseudocode for how to play the game on the micro:bit.
+>Example pseudocode:<br/>
+On button A press: choose random number from 0-2
+If random number = 0, then display rock icon,
+Else if random number = 1, then display paper icon, 
+Else display scissors icon.
+* Point out that because there are only three possibilities, we don’t need to do a separate check to see if random number = 2. So we just use an else.
+
+## micro:bit
+* Working from the specifications, have students work in pairs to try to code a Rock Paper Scissors game on their own.
+* If students get stuck, there is a tutorial at https://pxt.microbit.org/projects/rock-paper-scissors (steps 1 through 4), that leads students step-by-step through the process of coding a working rock paper scissor game for their micro:bit.
+* Let them play the game against their program.
+
+## Ideas for Mods
+* Add a way to keep score: Steps 5 through 7 in the tutorial
+* Mod the game to use different images or to add more options like ‘Rock Paper Scissors Lizard Spock’, Step 8 in the tutorial
+
+See the activity online: [**Rock, paper, scissors**](https://makecode.microbit.org/_A6Xbepc3w4uu)