Update front page docs to include Python 3.9 support

[nicholashughes]

Context

It appears Zappa supports supports 3.9 but front page doesn't mention that. This made me think twice about using this wonderful framework. Maybe I am wrong?

Also, when filing this Issue there is this statement in the issue generator itself:
"<!- Also, please make sure that you are running Zappa from a virtual environment and are using Python 3.6/3.7/3.8 >"
and maybe that should include 3.9 as well.

Expected Behavior

The front page docs should include Python 3.6/3.7/3.8/3.9

Actual Behavior

The front page docs mention Python 3.6/3.7/3.8/ but it looks like Zappa supports 3.9

Possible Fix

Change this sentence
Before you begin, make sure you are running Python 3.6/3.7/3.8
to
Before you begin, make sure you are running Python 3.6/3.7/3.8/3.9
as it now looks like Zappa supports 3.9

[thommor]

This has now been resolved