Requested changes to Carbon overview

[geoffromer]

(Prior discussion: #83 (comment))

In docs/design/README.md, I think the section titled "Carbon <-> C/C++ interoperability" should be renamed to something like "Bidirectional C++ interoperability": I think it puts more emphasis on the bidirectionality, without requiring the "<" HTML entity (or untypable "↔" character, as with prior drafts"), and without the redundant mention of "Carbon". The same applies to the title of docs/design/interoperability/template.md.

[jonmeow]

This has been addressed in #83