From OFLB

A fontlog gives users of your typeface useful information that might be hard to find elsewhere. You should add a fontlog to any typeface that you add to the Open Font Library, especially those licensed with the Open Font License. If you don’t add a fontlog the system will show a reminder.

Here are our suggestions for what your fontlog should contain.

The name and contact details of the original designer

This gives people who would like to contribute to the development of the existing typeface a way to contact you.

A description of how and why the typeface came into being

What need does it serve? Was it commissioned or did you create it to satisfy your own needs? You could link to an article or blog post about the project.

Whenever an existing font is amended, a list of the changes and additions

Give a summary of what the changes were, who did them, and how they can be contacted. List the most recent changes first. Remember that if you change an OFL licensed font that you did not create, you must choose a new name for the whole typeface family and rename your files to reflect that change.

In general, think about what the next user of the typeface might like to know. You do not have to list everything that you added, though.

The OFL FAQ provides an example fontlog you can copy and paste to get started.