update readme with benchmarks

[basaran]

Hello,

In case you would like to update the README with some benchmarks, I compared them as follows.

--
-- AMD Ryzen 3400G, PG13, WSL Ubuntu 20.04
--

-- Generation

-- uuid-ossp 
-- "Execution Time: 2712.880 ms"
explain analyze SELECT uuid_generate_v4()
FROM generate_series(1, 499000) t(i);

-- "Execution Time: 5083.430 ms"
explain analyze SELECT ulid_create()
FROM generate_series(1, 499000) t(i);

-- "Execution Time: 5927.542 ms"
explain analyze SELECT generate_ulid()
FROM generate_series(1, 499000) t(i);

-- Inserts

-- INSERT 0 499000 uuid_generate_v4()
-- Query returned successfully in 6 secs 748 msec.

-- INSERT 0 499000  ulid_create()
-- Query returned successfully in 8 secs 529 msec.

-- INSERT 0 499000 generate_ulid()
-- Query returned successfully in 12 secs 99 msec.