Improve README

[Jan Waś]

The README should be focused on users, not developers. For devs, we might add a DEVELOPER.md file.

[Jan Waś]

There should be examples how to incorporate time as a filter ($__timeFilter) in the query.

[Veera Pirla]

We should have examples for all macros

[Jan Waś]

I just tested macros and they work, this is great.

Example query:

select $__timeGroup(created_at,'1w') as date, to_milliseconds(avg(merged_at - created_at)) / 3600000 as avg_ttm_hours
from hive.v2.unique_pulls
where $__timeFilter(created_at)
group by 1
order by 1

We might be able to reuse most of this as docs: https://grafana.com/docs/grafana/latest/datasources/postgres/

[Jan Waś]

Also improve the getting started section. It should not assume users have built the plugin locally, instead use the GF_INSTALL_PLUGINS environment variable, like:

docker run -d -p 3000:3000 --name=grafana \
  -e "GF_INSTALL_PLUGINS=trino-datasource" \
  grafana/grafana-oss