Add support for recurring tasks ("rec:" keyword)

README.markdown

@@ -209,6 +209,26 @@ Defaults values are:
 
 For more information on the available flags see `help :sort`
 
+## Recurrence
+
+By adding a `rec:` tag to your task, when you complete (`<LocalLeader>x`) or
+postpone (`<LocalLeader>p`) the task, a new recurrence will be created due after
+the specified amount of time.
+
+The format is:
+    `rec:[+][count][d|w|m|y]`
+
+Where:
+    d = days, w = weeks, m = months, y = years
+    The optional `+` specifies strict recurrence (see below)
+
+Examples:
+    *   `rec:2w` - Recurs two weeks after the task is completed.
+    *   `rec:3d` - Recurs three days after the task is completed.
+    *   `rec:+1w` - Recurs one week from the due date (strict)
+
+This is a non-standard but widely adopted keyword.
+
 ## Mappings
 
 By default todo-txt.vim sets all the mappings described in this section. To