The SwiftSample doesn’t build in this commit, but I need to sync with
head anyway so I’ll first merge v1.0.x into this branch and then debug.

It’s the magic sentence that allows pods dependent on RxLibrary to be
archived correctly by XCode.

It’s less than ideal, and seems arbitrary (why RxLibrary specifically?),
so we’ll try to produce a minimal case and open an issue with it in the
Cocoapods repo.

Pending:

- Tag BoringSSL repo.
- Tag gRPC repo.
- Build the plugin, zip it, and add it to the release.
- Push Cocoapods to trunk.

It was still linking to my pull request's podspec.

Because the sample protos were all in a flat directory, I hadn't noticed the issue.

It also clarifies the Cocoapods part for people new to Cocoapods, and links to the sample app for examples on how to use the generated library.

Let's fix this before everybody hates me.