add "u" Format Character microseconds

same as PHP style, extending milliseconds

Author: JohnRDOrazio

README.md

@@ -86,7 +86,8 @@ PHP Style Format Characters (such as those found [here](http://php.net/manual/en
 | *H* | 24-hour format of an hour with leading zeros             | *00* through *23* |
 | *i* | Minutes with leading zeros                               | *00* to *59*      |
 | *s* | Seconds with leading zeros                               | *00* to *59*      |
-| *v* | Milliseconds                                             | Example *654*     |
+| *u* | Microseconds                                             | Example: *654321* |
+| *v* | Milliseconds                                             | Example: *654*    |
 | *Timezone*         | ---                                       | ---               |
 | *e* | Timezone identifier                                      | *UTC*, *UTC+1*    |
 | *I* (capital i) | Whether the date is in daylight saving time  | *DST* if Daylight Savings Time, otherwise nothing  |