Rework CONTRIBUTING:BEPs section for schema-driven approach

[yarikoptic]

trafficstars

ATM it still talks about changing regexes etc. It should be

• prepare branch/PR against bids-specification with changes for the BEP
• follow instructions here and/or there which boil down to
  • convert adjusted schema: cd bids-specification; bst -v export --output src/schema.json (fix if/how needed)
  • use it to validate sample data <path to bids-validator install>/bids-validator-deno -s src/schema.json <path to dataset>