Add a note in README about O1 not 01

[l2aelba]

Add a big note (or bold / danger) about using optimizations

About it O not 0 (not zero)

[jakubpawlowicz]

Good point @l2aelba, can you perhaps send a PR?

[wenz]

I fell into this very trap, so I went ahead and made a suggestion in a PR.