Provide "How to run local test" document about sqlc itself for new contributors

[vvakame]

What do you want to change?

Hi, I have interest to #1505
So, I tried to pass tests on my local environment first.
If this project have some document to contributors, I think it's very helpful to join development 👍

Here is the what I have tried, it seems that the test could be succeeded using the following procedure with #3944.
I am not sure if this procedure is sufficient.

export PATH=$HOME/bin:$PATH

export POSTGRESQL_SERVER_URI="postgresql://postgres:mysecretpassword@localhost:5432/postgres?sslmode=disable"
export MYSQL_SERVER_URI="root:mysecretpassword@tcp(localhost:3306)/dinotest?multiStatements=true&parseTime=true"

$ make start
$ make sqlc-dev sqlc-pg-gen sqlc-gen-json test-json-process-plugin
$ make test

What database engines need to be changed?

No response

What programming language backends need to be changed?

No response