Add Chapter 6 python files and fix grammar issues in readme

thinkcspy/Chapter-6-Exercises/Python-files/problem11.py

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+# Runestone.Academy thinkcspy course
+# Chapter 6
+# Problem 11
+
+import turtle
+
+def drawStar(t, numSides, sideLength):
+    angle = 180 - 180/numSides
+    
+    t.up()
+    t.goto(sideLength/2,0)
+    t.down()
+    
+    for i in range(numSides):
+        t.left(angle)
+        t.forward(sideLength)
+
+def checkSides(tt, sides, length):
+    if sides >= 3 and sides % 2 != 0:
+        print("What a beautiful star!")
+        drawStar(tt, sides, length)
+    else:
+        print("Oops! Your star must have an ODD number of at least 3 sides. Let's try that again!")
+
+    
+wn = turtle.Screen()
+wn.bgcolor("tan")
+
+stella = turtle.Turtle()
+stella.color("purple")
+stella.pensize(2)
+stella.speed(0)
+
+a = int(input("Help Stella draw a star! How many points does the star have?"))
+b = int(input("How long is each side?"))        
+
+checkSides(stella, a, b)
+
+wn.exitonclick()