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.

[leshow]

This puts Building dora for development as a sub-header of Features, I think. My original structure was to have that build/run header that directed people how to build with docker, where you don't have to mess with sqlx first and then the source build afterwards.

If you want to restructure it a little so that the other sections are more or less intact, but the sqlx stuff is put in the top section so it's more visible (after the docker build and dora_log stuff), I'd be good with that change.

[coveralls]

coverage: 72.673% (+1.0%) from 71.654%
when pulling 30f8f23 on stappersg:sqlx_needed
into 11180d9 on bluecatengineering:master.