Document the OMOPCommonDataModel.CDM_VERSION constant. (#16)

* Document the `OMOPCommonDataModel.CDM_VERSION` constant.

* Fix the doctests

Author: DilumAluthge

docs/src/version.md

@@ -10,3 +10,11 @@ corresponding version of the Common Data Model (CDM).
 | OMOPCommonDataModel.jl | CDM    |
 | ---------------------- | ------ |
 | 0.1.0                  | 6.0.0  |
+
+In order to see the current CDM version, use the `OMOPCommonDataModel.CDM_VERSION` constant:
+```jldoctest
+julia> using OMOPCommonDataModel
+
+julia> OMOPCommonDataModel.CDM_VERSION
+v"6.0.0"
+```