Create ALPS-UBER Mapping Guidance Document

[mamund]

write up the ALPS-UBER mapping guidance.

[mamund]

commited first draft here:
https://rawgit.com/alps-io/misc-docs/master/alps-to-uber.html

[mamund]

the current doc pattern focuses on three ALPS items:

• descriptor.id
• descriptor.name
• desdriptor.type

what about these?

• descriptor.rt
• link.rel
• doc.value
• ext

we might want to consider a standardized format that lists all the ALPS items and allows each mapping guide to indicate whether there is a "map" between ALPS and the target (maybe in a table form?).