Update CONTRIBUTING.md

+Issue is closed after pull request number is specified
Update AbsenceController's ExportAbsenceToExel + unitTests
Update ExportAbsenceToExel() And unitTests
Merge pull request #26 from Bohdan-Tereta/master

Fixed issues #23, #24, #27
Merge pull request #31 from AnnaStorozhenko/master

Solved #18: ExportAbsenceToExel() and AbsenceControllerTests modified
Issue # 10 BT Destination responsible manager to be informed about new BT fixed