Change project stage in README to "production read".

[Potherca]

This MR changes the badge for "Project Stage" to "Production Ready" (as the project has reached a v1). A link to the page which explains the rational behind the badge is also added.

Question to be aswered: Should we update the badge or does it make more sense to remove it? Orignally it was added to make clear (in the "swag" section) that the project was not yet production ready, as not everyone seems to understand what v0 versions seem to mean. However, as the default assumption seems to be that any project is production ready (unless otherwise stated) is the badge still needed?

[jrfnl]

@Potherca IMO the use of semver takes care of this, so the badge is not needed and can be removed. If people don't understand semver, they should educate themselves.

[Potherca]

Closed in favor of #204