Add ./skip convenience script; document SkipRunner

[dfabulich]

Skip Pull Request Checklist:

• REQUIRED: I have signed the Contributor Agreement
• REQUIRED: I have tested my change locally with swift test
• OPTIONAL: I have tested my change on an iOS simulator or device
• OPTIONAL: I have tested my change on an Android emulator or device

[marcprux]

We already have one of these in scripts/skip.local, which does a couple more things. Notably, it sets SKIP_COMMAND_OVERRIDE which will cause any self-forking commands to run the correct local build of Skip (e.g., when you run skip.local export, a Gradle build for Fuse projects that forks skip android build … will also use the local build).

[dfabulich]

Closing in favor of #170