Update model.md

Denzell Barnett · web-flow · commit 987c324833e9 · 2019-01-09T10:18:33.000-05:00
#49
diff --git a/doc/model.md b/doc/model.md
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-TODO model description
+# Masses and Springs: Basics - model description
+This simulation is a subset of the Masses and Springs simulation. The model is derived from MAS and is consistent with that sim. Energy values are not included in MASB.
+
+For details on the model for Masses and Springs, see https://github.com/phetsims/masses-and-springs/blob/master/doc/model.md.
