Document requirement to use same compiler for cargo-pgx and Toolkit

[syvb]

The same compiler must be used to compile cargo-pgx and Toolkit, or bad things (undefined behaviour) might happen. See pgcentralfoundation/pgrx#687 for why this is the case.

This PR updates the installation documentation in the README to indicate this. I split the command in two since you only need to reinstall cargo-pgx, there's no need to re-run cargo pgx init.

[rtwalker]

I split the command in two since you only need to reinstall cargo-pgx, there's no need to re-run cargo pgx init.

Considering this, do you want to make them two separate code blocks with the blurb about rust and cargo-pgx in between? Could also add something explaining that "there's no need to re-run cargo pgx init" after the second block for good measure

[rtwalker]

Thanks!

[syvb]

bors r+

[bors]

Build succeeded:

• Clippy Test
• Test Postgres (12)
• Test Postgres (13)
• Test Postgres (14)