add intro / explanations

[samuelweiler]

I might find it easier to understand this document if it included some explanatory and context-setting text - an introduction section. Consider expanding all domain-specific acronyms when first used (as requested for one case in https://github.com/w3c/imsc-hrm/issues/26)

[palemieux]

@samuelweiler Do you mean something like the explainer?

[palemieux]

@samuelweiler See #43