Update readme with upgrade instructions

[joncojonathan]

It wasn't clear to me how to upgrade, so I guessed on my Ghost instance and that seems to have worked. Added upgrade instructions to help others.

[joncojonathan]

Hello,

If this is still valid, please could it be merged? It may help others.

Many thanks :) .

[minimaluminium]

Hey @joncojonathan, sorry about the huge delay! Would you be able to update the download links in both installation and upgrade instructions?

Lyra's default branch isn't master anymore. The new download link would be https://github.com/TryGhost/Lyra/archive/refs/heads/main.zip 🙂

[minimaluminium]

@joncojonathan Thank you :)