Added policy regarding vacation

### Notes 

* Four weeks should be 28 days of vacation, because we do not distinguish between weekdays and weekends.
* I would make a section of the pay spreadsheet for vacation days taken per pay period.
* Half-days don't make sense now but might later. Smaller increments seem silly, though really any decimal portion of a day will be easy to calculate.
* I put it as optional to declare vacation way ahead and assign it to certain days, but we could also make things more required and just waive requirements as necessary.
  * Requiring assigning to certain days might be a good idea, otherwise it won't really feel like vacation.
  * I might like to change it so that you are required to declare a week in advance, or something like that. And we could waive as necessary.
* `days elapsed in year` will probably functionally be calculated as:
  `(number of current pay period) * 28`


### Please comment on

Any changes you would like to make, especially the things I wasn't sure about, as noted above.