Document dynamic destinations for avro

I've seen several folk struggle with writing Avro files to dynamic locations which I think might be a good addition to the pipelines patterns. 
 
An example showing how it may be done is in [this example](https://github.com/gbif/pipelines/blob/master/pipelines/export-gbif-hbase/src/main/java/org/gbif/pipelines/hbase/beam/ExportHBase.java#L81)

Notably it is not clear why this is necessary
```
.withDestinationCoder(StringUtf8Coder.of())
```

Imported from Jira [BEAM-7517](https://issues.apache.org/jira/browse/BEAM-7517). Original Jira may contain additional context.
Reported by: timrobertson100.