Add sample QASP to sample contract language

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Add a QASP to the sample contract language for 21st Century IDEA at https://designsystem.digital.gov/documentation/sample-contract-language/.

For example, consider adding an H2 section above Accessibility requirements with something like the following copy.

Include a surveillance plan to assess quality

Per the best practice to “require demos, not memos,” incorporate a quality assessment surveillance plan (QASP) into your solicitation. See the sample QASP in 18F's De-risking Guide for more details on deliverables, performance standards, acceptable quality levels, and methods of assessment to include in the plan.