docs: update readme in openapi-core

[tatomyr]

What/Why/How?

Config is indeed required when linting from openapi-core.

Security

• Security impact of change has been considered
• Code follows company security practices and guidelines

[changeset-bot]

⚠️ No Changeset found

Latest commit: 99ecbf3

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[github-actions]

Command	Mean [ms]	Min [ms]	Max [ms]	Relative
redocly lint packages/core/src/benchmark/benches/rebilly.yaml	965.4 ± 16.5	942.1	994.0	1.00
redocly-next lint packages/core/src/benchmark/benches/rebilly.yaml	965.9 ± 30.9	932.7	1029.2	1.00 ± 0.04

[github-actions]

Coverage report

St.❔	Category	Percentage	Covered / Total
🟡	Statements	79.44%	7039/8861
🟡	Branches	68.46%	2937/4290
🟡	Functions	75.16%	1150/1530
🟡	Lines	79.81%	6714/8412

Test suite run success

1340 tests passing in 193 suites.

Report generated by 🧪jest coverage report action from 99ecbf3