fixed pdf link

Removed OBSERVATION_PERIOD_START_DATETIME and OBSERVATION_PERIOD_END_…

…DATETIME from DDLs

cdm v5.3.1 visit_detail documentation fix

Typo fix

Merge pull request OHDSI#103 from gowthamrao/patch-1

Update OBSERVATION_PERIOD.md

Update README.md

Updating pdf documentation to reflect v5.1.1.

Update README.md

Edited Readme and removed pdf to v5 schema

New cost table instead of individual cost tables