Add Use Argument Indexes as a postgres til.

postgres/use-argument-indexes.md

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+# Use Argument Indexes
+
+In Postgres, each of the arguments you specify in a `select` statement has a
+1-based index tied to it. You can use these indexes in the `order by` and
+`group by` parts of the statement.
+
+Instead of writing
+
+```sql
+select id, updated_at from posts order by updated_at;
+```
+
+you can write
+
+```sql
+select id, updated_at from posts order by 2;
+```
+
+If you want to group by a table's `type` and then order by the counts from
+highest to lowest, you can do the following
+
+```sql
+select type, count(*) from transaction group by 1 order by 2 desc;
+```