Documentation on upgrading to 0.5?

Hi @antonmi ,

First of all, thank you so much for bringing espec and espec_phoenix to the ecosystem!

I have upgraded to v0.5 and as expected most of my test cases no longer work. Some are easier to transition to the "new approach", for instance I figured that instead of doing `be_redirection`, we should now use `html_response/2`, etc, however, not all the API changes are obvious, especially when the previous versions provided us with a lot of helper functions (for better or for worse).

What I am curious about is a smoother upgrade path for everyone. For example, if I have this before:

``` elixir
it do: should have_in_flash(success: "Signed up successfully.")
```

Do I now do this instead?

``` elixir
it do: assert get_flash(:success) == "Signed up successfully."
```

And if I had this before:

``` elixir
it do: should render_template("edit.html")
```

What is the equivalent new approach?

Thanks once again for the great library! :)