Add link to packages list

[Beliavsky]

At https://fpm.fortran-lang.org/registry/index.html, in the section

Note
The online registry as part of fpm is currently under active development and the online documentation might not be up-to-date.

I suggest adding a link to my list of packages. It could read

Note
The online registry as part of fpm is currently under active development and the online documentation might not be up-to-date. An unofficial list of about 400 FPM packages is here.