Enhance docstring with reference link

Updated docstring to include a reference link for the velocity addition formula.

Author: cclauss

physics/relativistic_velocity_summation.py

@@ -1,12 +1,16 @@
-# The relativistic velocity summation formula calculates the combined velocity v2 of
-# an object moving at speed v1 relative to a frame that is itself moving at velocity v
-# relative to an observer. I take the last one to be strictly lower than the speed of
-# light.
-# The formula is v2 = (v1 + v)/(1 + v1 * v / c**2)
-# v1 - speed of the object relative to a moving frame
-# v - speed of the moving frame
-# c - speed of light in the vacuum
-# v2 - speed of the object relative to an observer
+"""
+The relativistic velocity summation formula calculates the combined velocity v2 of
+an object moving at speed v1 relative to a frame that is itself moving at velocity v
+relative to an observer. I take the last one to be strictly lower than the speed of
+light.
+The formula is v2 = (v1 + v)/(1 + v1 * v / c**2)
+v1 - speed of the object relative to a moving frame
+v - speed of the moving frame
+c - speed of light in a vacuum
+v2 - speed of the object relative to an observer
+
+https://en.wikipedia.org/wiki/Velocity-addition_formula
+"""
 
 c = 299792458