C++ interoperability goals (#175)

Related threads:

-  From austern, [Initial draft of C++ Interoperability principles doc #62](#62)
-  From me, [Carbon <-> C/C++ interoperability #80](#80)
    - [Doc](https://docs.google.com/document/d/1va8VgvDdA966WG3znJyUrlComYqNfNBV7__hUd9XxxU/edit)
    - [Ideas topic](https://forums.carbon-lang.dev/t/draft-carbon-c-c-interoperability/77)
    - [RFC topic](https://forums.carbon-lang.dev/t/rfc-carbon-c-c-interoperability/89)
- From chandlerc, [Interop implementation strategies](https://forums.carbon-lang.dev/t/interop-implementation-strategies/108)

For this PR:

- [RFC topic](https://forums.carbon-lang.dev/t/rfc-c-interoperability-goals-175/156)
- [Decision topic](https://forums.carbon-lang.dev/t/request-for-decision-c-interoperability-goals/171)
- [Decision announcement](https://forums.carbon-lang.dev/t/accepted-c-interoperability-goals/175)
- [Decision PR](#200)

Author: jonmeow

proposals/README.md

@@ -49,5 +49,6 @@ request:
     -   [Decision](p0143_decision.md)
 -   [0149 - Change documentation style guide](p0149.md)
     -   [Decision](p0149_decision.md)
+-   [0175 - C++ interoperability goals](p0175.md)
 
 <!-- endproposals -->