Add README to update_responses to document downloads.json etc

Currently, we use the tag link using the following format for the Github release link, as follow: https://github.com/mullvad/mullvad-browser/releases/tag/mullvad-browser-102.11.0esr-12.0-1-build1

This is inconvenient for:

• downstream packagers using the tag to download from the GitHub page new releases
• dynamically linking our changelog on our website
• generally being able to look at the release version from the link

This is why, we have changed the release tags to the following format: https://github.com/mullvad/mullvad-browser/releases/tag/12.0.6

This needs to be updated when generating the XML update responses.

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Additional considerations (Nice to have if trivial to add)

• For now, we're manually adding this tag to the branch pushed on Github. If this can someone get added automatically, then it's a win.
• https://cdn.mullvad.net/browser/update_responses/update_1/release/downloads.json actually contains a tag, which is not present on the branch moved to our Github repository. Could this tag be changed to the same format XX.X.X?

EDIT: The tag referenced in downloads.json is not self-evident as to what it is for, we should add some small documentation there for users that go looking for it.