Refactored frequency enum to allow full decoupling of recurrence tran…

…sformations

src/main/java/net/fortuna/ical4j/transform/recurrence/Frequency.java

@@ -1,5 +1,26 @@
 package net.fortuna.ical4j.transform.recurrence;
 
+/**
+ * Represents the possible expansion rules used to generate recurrences.
+ *
+ * From RFC5545:
+ *
+ * <pre>
+ *       The FREQ rule part identifies the type of recurrence rule.  This
+ *       rule part MUST be specified in the recurrence rule.  Valid values
+ *       include SECONDLY, to specify repeating events based on an interval
+ *       of a second or more; MINUTELY, to specify repeating events based
+ *       on an interval of a minute or more; HOURLY, to specify repeating
+ *       events based on an interval of an hour or more; DAILY, to specify
+ *       repeating events based on an interval of a day or more; WEEKLY, to
+ *       specify repeating events based on an interval of a week or more;
+ *       MONTHLY, to specify repeating events based on an interval of a
+ *       month or more; and YEARLY, to specify repeating events based on an
+ *       interval of a year or more.
+ * </pre>
+ *
+ * See https://tools.ietf.org/html/rfc5545#section-3.3.10 for more details.
+ */
 public enum Frequency {
     SECONDLY, MINUTELY, HOURLY, DAILY, WEEKLY, MONTHLY, YEARLY
 }