Add Chapter 6 python files and fix grammar issues in readme

thinkcspy/Chapter-6-Exercises/Python-files/problem15.py

@@ -0,0 +1,18 @@
+# Runestone.Academy thinkcspy course
+# Chapter 6
+# Problem 15
+
+def myPi(iters):    
+    # Calculate an approximation of PI using the Leibniz
+    # approximation with iters number of iterations
+    
+    # your code here  
+    apprPi = 0
+    
+    for i in range(iters):
+        term = (-1)**i/(2*i+1)
+        apprPi += term
+    
+    return apprPi * 4
+        
+print(myPi(1000000))