Update clickhouse.md with callout to invalid utf-8 (#4527)

Mentioning that users may first need to cast strings with toValidUTF8 function to avoid any issues with string columns that happen to contain invalid utf-8 in certain rows

docs/docs/reference/olap-engines/clickhouse.md

@@ -112,5 +112,6 @@ Note that you must `cd` into the Git repository that your project was deployed f
 
 ## Additional Notes
 
-- At the moment, Rill does not officially support modeling with ClickHouse. If this is something you're interested in, please [contact us](../../contact.md).
-- For dashboards powered by ClickHouse, [measure definitions](/build/dashboards/dashboards.md#measures) are required to follow standard [ClickHouse SQL](https://clickhouse.com/docs/en/sql-reference) syntax.
+- At the moment, we do not officially support modeling with ClickHouse. If this is something you're interested in, please [contact us](../../contact.md).
+- For dashboards powered by ClickHouse, [measure definitions](/build/dashboards/dashboards.md#measures) are required to follow standard [ClickHouse SQL](https://clickhouse.com/docs/en/sql-reference) syntax.
+- Because string columns in ClickHouse can theoretically contain [arbitrary binary data](https://github.com/ClickHouse/ClickHouse/issues/2976#issuecomment-416694860), if your column contains invalid UTF-8 characters, you may want to first cast the column by applying the `toValidUTF8` function ([see ClickHouse documentation](https://clickhouse.com/docs/en/sql-reference/functions/string-functions#tovalidutf8)) before reading the table into Rill to avoid any downstream issues.