add docstring for credit_balance()

src/DegreePlanAnalytics.jl

@@ -140,7 +140,20 @@ function requisite_distance(plan::DegreePlan)
     return plan.metrics["requisite distance"] = distance
 end
 
-#TODO: write docstring, write tests
+"""
+    credit_balance(plan::DegreePlan)
+
+For a given degree plan `plan`, this function computes and returns the credit balance among the terms in a degree plan.  A 
+score of `0` indicates perfect balance, i.e., each term in the degree plan has the same number of credit hours. As a degree plan
+becomes more imbalanced, in terms of the number of credit hours in each tersm, the credit balance grows larger. If ``t_j`` 
+denotes the number of credit hours in term ``j``, ``j = 1 \ldots m,`` then the credit balance of degree plan `p`, denoted by
+``cb^p is given by:
+
+```math
+cb^p = \\sum_{i=1}^{m-1}\\abs(t_j - t_{j+1})
+```
+
+"""
 function credit_balance(plan::DegreePlan)
     bal = 0
     for i in 1:length(plan.terms)-1