README: sqlx is needed for cargo build

[stappersg]

Re-ordered build instructions. Because cargo build fails when there was previously no sqlx database create and sqlx migrate run.

[stappersg]

( Oh, the diff looks horrible. )

Reason for this merge request is that

git clone https://github.com/bluecatengineering/dora.git
cd dora
cargo build

yields many build errors.

Those errors are gone after

sqlx database create
sqlx migrate run

The README.md has now the SQLX part before cargo build works part.