Update conferences.md
Merge pull request #25 from thoughtbot/am-conference-signup

Update conferences.md
Revise Performance Review Questions
Separate the permission-seeking and time-tracking pieces of PTO

* Move the leave of absence text into the general overview
* Clarify that permission is asked/announced via email to admin
* Clarify that future time off is indicated via Forecast and calendar
* Clarify that past time off is tracked via Harvest