Clone this plugin into your codebase.

Add the plugin to PostGraphile via `--append-plugins` (CLI) or `appendPlugins: [...]`.

Add a comment on the relevant constraint and/or table to enable upsert:

```sql
comment on table my_table is E'@upsert';
-- or
comment on constraint my_table_pkey of table my_tabls is E'@upsert';
```