This is the template in Documenting architecture decisions - Michael Nygard. You can use adr-tools for managing the ADR files.
In each ADR file, write these sections:
What is the status, such as proposed, accepted, rejected, deprecated, superseded, etc.?
What is the issue that we're seeing that is motivating this decision or change?
What is the change that we're proposing and/or doing?
What becomes easier or more difficult to do because of this change?