Document how to add a new project

Signed-off-by: Michael Opdenacker <michael.opdenacker@free-electrons.com>

README.md

@@ -15,7 +15,7 @@ duplicating work and data. It has a straightforward data structure
 Requirements
 ------------
 
-* >= Python 3.5
+* Python >= 3.5
 * The Jinja2 and Pygments Python libraries
 * Berkeley DB (and its Python binding)
 * Exuberant Ctags
@@ -88,9 +88,7 @@ directory with a specific structure:
     * repo
 
 It will then generate the other two variables upon calling the query
-command. For now, three projects are hard-coded into the shell script
-(to handle version grouping and display): Linux, U-Boot, Busybox,
-Musl and Zephyr.
+command.
 
 Here is an example configuration for Apache:
 
@@ -115,4 +113,92 @@ Here is an example configuration for Apache:
 
 Don't forget to enable cgi and rewrite support with `a2enmod cgi rewrite`.
 
-Note: this documentation applies to version 0.2 of Elixir.
+Supporting a new project
+------------------------
+
+Elixir has a very simple modular architecture that allows to support
+new source code projects by just adding a new file to the Elixir sources.
+
+Elixir's assumptions:
+
+* Project sources have to be available in a git repository
+* All project releases are associated to a given git tag. Elixir
+  only considers such tags.
+
+First make an installation of Elixir by following the above instructions.
+See the `projects` subdirectory for projects that are already supported.
+
+Once Elixir works for at least one project, it's time to clone the git
+repository for the project you want to support:
+
+> cd /srv/git
+> git clone --bare https://github.com/zephyrproject-rtos/zephyr
+
+Now, in your `LXR_PROJ_DIR` directory, create a new directory for the
+new project:
+
+    cd $LXR_PROJ_DIR
+    mkdir -p zephyr/data
+    ln -s /srv/git/zephyr.git repo
+    export LXR_DATA_DIR=$LXR_PROJ_DIR/data
+    export LXR_REPO_DIR=$LXR_PROJ_DIR/repo
+
+Now, go back to the Elixir sources and test that tags are correctly
+extracted:
+
+> ./script.sh list-tags
+
+Depending on how you want to show the available versions on the Elixir pages,
+you may have to apply substitutions to each tag string, for example to add
+a `v` prefix if missing, for consistency with how other project versions are
+shown. You may also decide to ignore specific tags. All this can be done
+by redefining the default `list_tags()` function in a new `project/<projectname>.sh`
+file. Here's an example (`projects/zephyr.sh` file):
+
+    list_tags()
+    {
+        echo "$tags" |
+        grep -v '^zephyr-v'
+    }
+
+Note that `<project_name>` **must** match the name of the directory that
+you created under `LXR_PROJ_DIR`.
+
+The next step is to make sure that versions are classified as you wish
+in the version menu. This classification work is done through the
+`list_tags_h()` function which generates the output of the `./scripts.sh list-tags -h`
+command. Here's what you get for the Linux project:
+
+    v4 v4.16 v4.16
+    v4 v4.16 v4.16-rc7
+    v4 v4.16 v4.16-rc6
+    v4 v4.16 v4.16-rc5
+    v4 v4.16 v4.16-rc4
+    v4 v4.16 v4.16-rc3
+    v4 v4.16 v4.16-rc2
+    v4 v4.16 v4.16-rc1
+    ...
+
+The first column is the top level menu entry for versions.
+The second one is the next level menu entry, and
+the third one is the actual version that can be selected by
+the menu.
+
+If the default behavior is not what you want, you will have
+to customize the `list_tags_h` function.
+
+You should also make sure that Elixir properly identifies
+the most recent versions:
+
+> ./script.sh get-latest
+
+If needed, customize the `get_latest()` function.
+
+You are now ready to generate Elixir's database for your
+new project:
+
+> ./update.py
+
+You can then check that Elixir works through your http server.
+
+Note: this documentation applies to version 0.3 of Elixir.