Describe the special characters in the readme

Author: Gilthoniel

README.md

@@ -23,3 +23,13 @@ This package implements the specification found in [Wikipedia](https://en.wikipe
 | Month        | 1-12           | , - * /                    |
 | Day of week  | 0-6 or SUN-SAT | , - * ? / L #              |
 | Years        | 1-9999         | , - * /                    |
+
+#### Special characters
+
+* `*` can be used to select all values within a field (e.g. every second, every minute, ...)
+* `?` can be used for no specific value in the day field or the week day field.
+* `-` can be used to specify an inclusive range (e.g. `1-3` means values 1, 2, and 3).
+* `,` can be used to list values (e.g. `1,2,3` means values 1, 2 and 3).
+* `/` can be used to specify an interval (e.g. `1/5` means values 1, 6, 11, 16, etc...).
+* `L` when used in the month field specifies the last day of the month and Saturday when used in the week day field. Using a digit before the character in the week day field specifies the nth last week day of the month (e.g. `1L` for the last Monday of the month). An offset can also be used for the month field (e.g. `L-2` for the second last day of the month).
+* `#` can be used to specify the nth week day of the month (e.g. `6#3` for the third (`3`) Friday (`6`) of the month).