Make a note about sqlx macros opt level

[CosmicHorrorDev]

Changing this build setting made cargo check over twice as fast on a project I work on. I didn't see this mentioned anywhere in the documentation for this project, so I figured it was worth adding a note

[abonander]

I've tried setting RUSTFLAGS=-C opt-level=3 in the past but it didn't really seem to help despite forcing a recompile of the entire dependency tree. I think I tried this exact trick as well, in sqlx-macros itself, and it also didn't really help. I'll try it again though.

It may also depend on whether you're compiling against a database running on the same machine or not, as when I accidentally compile against a hosted database in the cloud rather than a local machine, it easily takes several orders of magnitude longer, likely due to hitting more of Tokio's I/O driver code which is also not optimized when building in debug mode.

[CosmicHorrorDev]

It's possible this won't make a significant difference after #1684. The situation where I noticed a large speedup was from cargo check times with a lot of queries and a relatively large sqlx-data.json file, but testing again after #1864 was applied led to less than a 0.2 second difference. Using a remote db and local db both show basically the same time.

If you want you can close this in favor of #1684. I figured there would be a significant difference when using a remote db too, but that doesn't seem to be the case