[gnome-builder] doc: start on an xdg-app readme

[Christian Hergert <chergert src gnome org>]

commit eb380e6e8efac6bed3d762cd00d8fc847f19a2de
Author: Christian Hergert <chergert redhat com>
Date:   Tue Oct 27 11:30:04 2015 -0700

    doc: start on an xdg-app readme

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+# Building an Xdg-App
+
+## Get latest xdg-app
+
+Install xdg-app from `git://anongit.freedesktop.org/xdg-app/xdg-app`.
+It contains a few fixes required for Builder.
+
+```sh
+jhbuild build xdg-app
+```
+
+## Install the GNOME SDK GPG key
+
+```sh
+wget http://sdk.gnome.org/keys/gnome-sdk.gpg
+gpg --import gnome-sdk.gpg
+rm gnome-sdk.gpg
+```
+
+## Get the GNOME SDK Runtime
+
+We need the SDK instead of the normal application runtime since it includes
+things like clang for us to use to compile.
+
+```sh
+xdg-app add-remote --user gnome-sdk http://sdk.gnome.org/repo/
+xdg-app install-runtime --user gnome-sdk org.gnome.Sdk 3.18
+```
+
+## Build the Xdg-App
+
+```sh
+make dist
+cd build/xdg-app
+./build.sh gnome-builder.def
+```
+
+## Install the Xdg-App
+
+We need to add the location where we built the application to our xdg-app
+repos.  Alternatively, you could export a single file and import that into your
+user xdg-app tree.
+
+I'm almost positive there is a better way to do this, but this will get you
+started.
+
+```sh
+xdg-app add-remote --user gnome-builder ./repo
+xdg-app install-app gnome-builder org.gnome.Builder
+```
+
+If you've already done the above, and you made a new build, you'll need to
+update the app in your xdg-app tree instead.
+
+```sh
+xdg-app update-app --user org.gnome.Builder
+```
+
+## Run the Xdg-App
+
+```sh
+xdg-app run org.gnome.Builder
+```
+
+If you want to look around in the runtime environment, you might consider
+using the following.
+
+```sh
+xdg-app run --devel --command=bash org.gnome.Builder
+```
+
+This will give you a shell as if you replaced the `gnome-builder` executable
+with `bash`.