README: update for 3.0 (#625)
README: update for 3.0 (#625)

This updates the README.md for features added to 3.0, and other cleanup.

- Remove github release badge (only tags matter).
- Update repository overview with new folders and better descriptions.
- Update required dcrd to 1.3 (testnet3).
- Clean up and clarify Upgrading Instructions.
- Add more about PostgreSQL tuning.
- Add System hardware requirements section.
- Update JSON API tables with new endpoints (block subsidy, tx vinfo, tx hex, tx decoded, address totals).
- fix typos, add tips about permissions and dcrd settings
- note experimental Insight API support
dcrdata/dcrdata,dcrdata/dcrdata,dcrdata/dcrdata,dcrdata/dcrdata,dcrdata/dcrdata