add ISODuration type for ISO8061 duration compatibility

README.md

@@ -1,3 +1,21 @@
 # clocktime
 
 clocktime provides a type that holds a calendar-independent time of day value in 24-hour format.
+
+clocktime provides basic types that extend Go's built-in time.Time to provide time-of-day and duration support.
+
+## `clocktime.ClockTime
+
+Holds calendar-independent time-of-day in 24-hour format.
+
+- Converts to and from `HH:MM:SS` format (subset of ISO8061)
+- Marshals to `[]byte` containing string `HH:MM:SS` for SQL
+- Marshals to `string` in format `HH:MM:SS` for JSON & gqlgen
+
+## `clocktime.Duration`
+
+Wraps time.Duration with prioritized ISO8061 support and opinionated marshalling.
+
+- Converts to and from ISO8061 format (utilises [sosodev/duration](https://github.com/sosodev/duration))
+- Marshals to `uint64` with nanosecond precision. Largest representatable duration is about 290 years, limited by underlying time.Duration type.
+- Marshals to `string` in ISO8061 format for JSON & gqlgen