Small QoL improvements

[Ben-Epstein]

First time running this library, two first things that came up and caused slight pains

1. instructions on how to install espeak (never heard of it)
2. fix list_models and also make it a static method so TTS.list_models() works as intended

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[eginhard]

The list_models() method is already fixed in our fork, available via pip install coqui-tts