Add validation steps for schema extensions

At a glance

• Identifier: #454
• Stage: RFC3: Accepted
• Champion: @IvanGoncharov
• PR: Add validation steps for schema extensions

Timeline

• Spec PR created on 2018-06-04 by IvanGoncharov
• Commit pushed: Add validation steps for schema extensions on 2018-06-04 by @IvanGoncharov

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Reported by @OlegIlyenko in https://github.com/facebook/graphql/issues/429#issuecomment-392990065 I think it should be added in upcomming release for consistency with other definition extensions.