Update README.md

#38 - Adds verbiage to the documentation regarding the limitations of using the `Duration` type.

Author: StevenRasmussen

README.md

@@ -12,7 +12,7 @@ When modelling, usage of the following NodaTime types are supported:
 * LocalDateTime
 * LocalDate
 * LocalTime
-* Duration
+* Duration (stored as `time` in SQL Server which limits this type to < 24 hours)
 
 When querying, standard operators are supported as well as a range of additional mappings from NodaTime properties/function to their SQL Server equivalents.
 
@@ -239,4 +239,4 @@ await this.Db.RaceResult
 * 3.1.1
   * Fix SqlDateTime overflow - [#13](/../../issues/13)
 * 3.1.0
-  * Sync version number with .Net Core 3.1.0
+  * Sync version number with .Net Core 3.1.0