Pverlay development documentation

I have a need to write an overlay (openldap-2.4.44) and wondered if there was any documentation for overlay development? I have examined the sources in contrib/slapd-modules/ and was hoping to locate something a bit more descriptive and maybe any must-do's and gotchas for development.

From a development environment perspective, is it advisable to use the

contrib/slapd-modules/ as a build location, using a Makefile based on one of the existing overlays and also the non-exposed header files (eg "portable.h") or not?

- Geoff