README: mention / suggest official manylinux images

[henryiii]

The official manylinux images support PyPy, so pointing that out as well as pointing out the differences would be very useful. Something like:

Pro's:

• Updated frequently with the latest dependencies, like Pip 21.3, etc.
• Not fighting docker for image quotas, weekly stable tagged releases
• Supports manylinux 2010 like this one, but also 2014 and 2_24 as well
• Some alternate architecture support (Aarch64)

Con's:

• Only supports the latest PyPy release
• Only provides the PyPy 3.7 variant.

I'm not sure what the long term plans are for this repo, if you want to focus on the official image instead, etc - that's up to you. But adding the above information would probably help users, and could even be considered a solution to #7, for example.