The developer's documentation for Mesquite version 0.98 is here:
It will unzip to a folder "development". It is designed to replace the folder Mesquite_Folder/docs/mesquite/development that appears in the base Mesquite system.
Documentation updated to apply to version 0.99 or later is not yet available. Contact Wayne Maddison (info@mesquiteproject.org) for details about developing for Mesquite.