add stable releases to be used with jitpack

[thombergs]

When using jitpack, you have to provide stable releases for the users, so they don't always take the latests "snapshot" version. It should be enough to create tags through the GitHub UI and provide information in the readme on how the user can find out which releases are available by pointing them to the tags list.

[jeysal]

A Jitpack badge can be used to display the latest available version.
See here for an example.