Upgrade 'Document Creation Information' according to the 2.1 spec

[yash-nisar]

Attributes to be added are:

• SPDX Identifier
• Document Name
• SPDX Document Namespace
• External Document References

See #71.

[krysnuvadga]

@yash-nisar it will be nice if the issues are created with more detailed descriptions to them. For " Upgrade 'Document Creation Information' according to the 2.1 spec" is not enough to describe the entire task