Ensure validity of code snippets in documentation and README

[svix-daniel]

Right now the code snippets in the documentation are marked as compile_fail. This is because behind the scenes, each snippet is wrapped in a synchronous function, when all the examples are async. There are most certainly ways around this, so we should fix this as soon as I have a spare moment.

Additionally @svix-onelson (in #1 (comment)) says that there's something we can do to have the same doc tests run for snippet in the README, which we should do if possible.