To create a new extension, add a subdirectory in extensions.
Then create a Makefile.am like the one in example, replacing
the EXTENSION_ID with the basename of your extension, which
must match the UUID in metadata.json.
If you need additional files, add them to EXTENSION_EXTRA.

Then modify extensions/Makefile.am and configure.ac. It should
be pretty self-explanatory.

Don't forget to add any translatable file to po/POTFILES.in, and
then you're done.