OpenTM2 Translators Reference
You can always find the most current version of the OpenTM2 Translators Reference, the definitive guide for translators and other users of OpenTM2, in the OpenTM2 GitHub repository (Static PDF).
OpenTM2 Developers Reference
The OpenTM2 Translators Reference contains extensive sections concerning issues relevant to developers – like API or Command-line usage. If you are a developer, look it up! (Static PDF).
OpenTM2 change log
For each new version, a changes.txt file is published at https://github.com/OpenTM2/opentm2-source/tree/master/changes. It lists all new features and bugfixes made in that version.