Add doc strings.

src/SimplePeriods.jl

 @enum Frequency Year Semester Quarter Month Week Business Day Undated
 
+"""
+Base year used to compute the `ordinal` field of a Period type.
+"""
 BaseYear = 1970
 
+"Composite type Period has fields `ordinal` and `frequency`."
 struct Period <: AbstractPeriod
+    """
+    Number of periods since year BaseYear.
+    """
     ordinal::Int64
+    """
+    Frequency of the period, possible values are:
+     - `Year`,
+     - `Semester`,
+     - `Quarter`,
+     - `Month`,
+     - ` Week`,
+     - `Day`, and
+     - `Undated` (no frequency)
+    """
     frequency::Frequency
 end
 
+#
+# Constructors
+#
+
+"""
+    Period(year)
+
+Constructor for periods with unspecified frequency.
+"""
+function Period(arg1::Integer)
+    Period(arg1, Undated)
+end
+
+"""
+    Period(year, frequency)
+
+Constructor for yearly periods, if second argument's value is `Year`, or periods with unspecified frequency if
+second argument's value is `Undated`.
+"""
 function Period(arg1::Integer, frequency::Frequency)
     if frequency == Year
         ordinal = arg1 - BaseYear
@@ -18,23 +54,29 @@ function Period(arg1::Integer, frequency::Frequency)
     Period(ordinal, frequency)
 end
 
+"""
+    Period(year, subperiod, frequency)
+
+Constructor for bi-annual (third argument's value is `Semester`), quarterly (third argument's value is `Quarter`),
+monthly (third argument's value is `Month`) or weekly (third argument's value is `Week`) periods.
+"""
 function Period(arg1::Integer, arg2::Integer, frequency::Frequency)
     if frequency == Year
         ordinal = arg1 - BaseYear
     elseif frequency == Semester
-        @assert(arg2 in 1:2)
+        @assert (arg2 in 1:2) "subperiod (semester) has to be equal to 1 or 2."
         ordinal = 2*(arg1 - BaseYear) + arg2 - 1
     elseif frequency == Quarter
-        @assert(arg2 in 1:4)
+        @assert (arg2 in 1:4) "subperiod (quarter) has to be an integer between 1 and 4."
         ordinal = 4*(arg1 - BaseYear) + arg2 - 1
     elseif frequency == Month
-        @assert(arg2 in 1:12)
+        @assert (arg2 in 1:12) "subperiod (month) has to be an integer between 1 and 12."
         ordinal = 12*(arg1 - BaseYear) + arg2 - 1
     elseif frequency == Week
-        @assert(arg2 <= weeksinyear(arg1))
+        @assert (arg2 <= weeksinyear(arg1)) "subperiod (week) has to be an integer between 1 and $(weeksinyear(arg1))."
         ordinal = numberofweeks(arg1, arg2)
-    elseif frequency == Undated
-        ordinal = arg1
+    else
+        error("Possible values for the last argument are `Month`")
     end
     Period(ordinal, frequency)
 end
@@ -106,9 +148,25 @@ function numberofweeks(y::Integer, w::Integer)
 end
 
 import Base.copy
+"""
+    copy(p)
+
+Return a copy of Period type p.
+"""
 copy(p::Period) = Period(p.ordinal, p.frequency)
 
 import Base.:+
+"""
+    +(p, k)
+
+Add k subperiods to Period type p.
+"""
 +(p1::Period, k::Integer) = Period(p1.ordinal + k, p1.frequency)
+
 import Base.:-
+"""
+    +(p, k)
+
+Substract k subperiods to Period type p.
+"""
 -(p1::Period, k::Integer) = Period(p1.ordinal - k, p1.frequency)