Created PGX Support (markdown)

PGX-Support.md

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+KSQL works on top of existing and battle-tested SQL driver implementations including `pgx`.
+
+Currently, KSQL supports 2 major versions of PGX:
+
+- PGX 4 with the adapter available at: github.com/vingarcia/ksql/adapters/pgx
+- PGX 5 with the adapter available at: github.com/vingarcia/ksql/adapters/pgx5
+
+Both adapters provide 2 constructors with the same names and signatures:
+
+- `New(ctx context.Context, connectionString string, config ksql.Config) (db ksql.DB, err error)`
+- `NewFromPgxPool(pool *pgxpool.Pool) (db ksql.DB, err error)`
+
+The first constructor hides most options, while the second constructor allows you to provide a fully configured `pgxpool` instance, meaning you can use all supported `pgxpool` configurations and still use KSQL on top of it.
+
+## PGX Special Types
+
+When using the `Query`, `QueryOne` and `QueryChunks` functions all arguments passed after the query as well as all the attributes of the input structs are directly forwarded to the underlying database adapter, in this case `pgx`. This means that you can actually use any supported `pgx`.
+
+The example query below also [available here][examples/pgxsupport] illustrates one possible use-case:
+
+[examples/pgxsupport]: https://github.com/VinGarcia/ksql/blob/master/examples/pgxsupport/main.go#L92
+
+```golang
+	// Find user iff user belongs to either team on the input list:
+	var user User
+	err = db.QueryOne(ctx, &user,
+		`SELECT u.*
+		FROM users AS u
+		JOIN team_members AS tm
+			ON u.id = tm.user_id
+		WHERE u.id = $1
+			AND tm.team_id = ANY($2)`,
+		userID,
+		[]int{1, 2, 42}, // Int slices are supported by PGX
+	)
+	if err == ksql.ErrRecordNotFound {
+		fmt.Println("Input user does not exist or does not belong to any of the provided teams")
+		return
+	} else if err != nil {
+		log.Fatalf("unexpected error: %s", err)
+	}
+
+	fmt.Printf("Found user: %+v\n", user)
+```