model notes added and implementation-notes created, see #19

doc/model.md

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+# Inverse Square Law Model
+
+This document describes the model for the all Inverse Square Law simulations.<br>
+@author Michael Barlow (PhET Interactive Simulations)
+
+## Coulomb's Law
+Coulomb's law  defines the force that two charges will exert upon each other.
+
+F = k( q1 * q2 / r<sup>2</sup>)
+
+where:
+
+F = the force on each charge<br>
+k = Coulomb's constant (8.99 x 10<sup>9</sup> Nm<sup>2</sup>C<sup>-2</sup>)<br>
+q1 = the charge *left* object<br>
+q2 = the charge *right* object<br>
+r = the distance between the charges
+
+## Gravitational Force
+Gravitation describes the attractive force between two point masses.
+
+F = G( m1 * m2 / r<sup>2</sup>)
+
+where:
+
+F = the force on each mass<br>
+G = the gravitational constant (6.674 x 10<sup>-11</sup> m<sup>3</sup>kg<sup>-1</sup>s<sup>-2</sup>)<br>
+m1 = the mass *left* object<br>
+m2 = the mass *right* object<br>
+r = the distance between the masses