Document on NTIA minimum requirements and how it applies to SPDX builder.

[AnnieJovitha]

Explain what are the minimum and why we decide to keep the value field the way we see it in SPDX output.

eg. like packageName , supplier field
ambiguity

PackageName: "org.springframework.boot/spring-boot-actuator "or " spring-boot-actuator"
PackageVersion: 2.1.1.RELEASE
PackageSummary: spring-boot-actuator
PackageHomePage: http://projects.spring.io/spring-boot/
ExternalRef: PACKAGE-MANAGER purl pkg:maven/org.springframework.boot/spring-boot-actuator@2.1.1.RELEASE