docs: Update instructions for extension additions for meson

The steps for adding a new extension are clearly different in meson,
so update the instructions accordingly. Don't bother with keeping
the existing autotools steps - supporting both build systems in
parallel is just temporary, autotools is on its way out.

Fixes https://gitlab.gnome.org/GNOME/gnome-shell-extensions/issues/31

HACKING.md

@@ -1,13 +1,13 @@
 ## Creating a New Extension
 
-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.
+To create a new extension, add a subdirectory in extensions. Then create
+a meson.build from the provided [template](extensions/meson.build.template).
+If you need additional sources, add them to extension_sources. Similarily add
+GSettings schemas to extension_schemas and other files to extension_data.
 
-Then modify extensions/Makefile.am and configure.ac. It should
-be pretty self-explanatory.
+Then modify the [toplevel Meson file](meson.build) to add the new
+extension name in the appropriate set (that is one of classic_extensions,
+default_extensions or all_extensions).
 
 Don't forget to add any translatable file to po/POTFILES.in, and
 then you're done.

extensions/meson.build.template

@@ -0,0 +1,8 @@
+extension_data += configure_file(
+  input: metadata_name + '.in',
+  output: metadata_name,
+  configuration: metadata_conf
+)
+
+# extension_sources += files('prefs.js')
+# extension_schemas += files(metadata_conf.get('gschemaname') + '.gschema.xml')