[gtk/docs-gtk-org] Document what this branch does

[Emmanuele Bassi <ebassi src gnome org>]

commit bebf063ff3d6d9ea12bdac35f8adab3f07699f50
Author: Emmanuele Bassi <ebassi gnome org>
Date:   Sat Jul 31 18:15:28 2021 +0100

    Document what this branch does

 README.md | 69 +++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++
 1 file changed, 69 insertions(+)
---
diff --git a/README.md b/README.md
index e69de29bb2..e232940e78 100644
--- a/README.md
+++ b/README.md
@@ -0,0 +1,69 @@
+GTK Documentation site
+======================
+
+This branch of the GTK repository is used to generate the content of the
+[docs.gtk.org](https://docs.gtk.org) website.
+
+Contents
+--------
+
+```
+.
+├── glib
+│   ├── gio
+│   ├── glib
+│   └── gobject
+├── gtk3
+│   ├── gdk
+│   └── gtk
+├── static
+└── subprojects
+    └── gi-docgen.wrap
+```
+
+The landing page is store inside the [`static`](./static) directory.
+
+The [`glib`](./glib) directory contains the introspection data and gi-docgen
+project files for GLib, GObject, and GIO.
+
+The [`gtk3`](./gtk3) directory contains the introspection data and gi-docgen
+project files for GTK3 and GDK3.
+
+The [`subprojects`](./subprojects) directory contains a Meson subproject for
+gi-docgen.
+
+How does this work
+------------------
+
+The CI pipeline for the main development branch of GTK builds the API
+references for the following projects:
+
+ - gdk
+ - gtk
+ - pango
+ - gdk-pixbuf
+
+The generated documentation is stored as an artifact inside GitLab.
+Additionally, the CI pipeline will use a pipeline trigger for the
+`docs-gtk-org` branch (the branch that contains the `README` file you are
+currently reading).
+
+The CI pipeline for the `docs-gtk-org` branch will:
+
+ - download the build artifacts
+ - extract the various API references
+ - run gi-docgen on the introspection data of the following projects:
+   - glib
+   - gtk3
+ - publish the static landing page
+ - publish all the API references
+
+ Notes
+ -----
+
+ The token for the pipeline trigger is stored in the `PAGES_TRIGGER_TOKEN`
+ environment variable, which is exposed to the CI pipelines of the GTK
+ project.
+
+ Only the `docs-gtk-org` branch can publish the contents of the docs.gtk.org
+ website.