Add section about current releases

This is not ideal as the releases are hardcoded, but I think is better
than nothing until we got a solution to automate this

doc/source/index.rst

@@ -18,6 +18,7 @@ Later sections provide detailed information on BuildStream internals.
    :maxdepth: 1
 
    main_about
+   main_releases
    main_install
    main_using
    main_core

doc/source/main_install.rst

+.. _main_install:
+
 Install
 =======
 This section covers how to install BuildStream onto your machine, how to run

doc/source/main_releases.rst

+Releases
+========
+BuildStream follows the Semantic Versioning Convention `(SemVer) <https://semver.org/>`_
+for releases and development snapshops.
+
+For example, for a given version number ``X.Y.Z``
+ * The ``X.<even number>.*`` versions are stable releases
+ * The ``X.<odd number>.*`` versions are development spanshots.
+
+Therefore, please look for the latest tag in the latest release branch to ensure
+you're getting the latest release
+
+Current release branches:
+ * `bst-1.2 (latest) <https://gitlab.com/BuildStream/buildstream/commits/bst-1.2>`_
+     * Latest release: `1.1.6 <https://gitlab.com/BuildStream/buildstream/tags/1.1.6>`_
+ * `bst-1.0 (deprecated) <https://gitlab.com/BuildStream/buildstream/commits/bst-1.0>`_
+
+Check the :ref:`BuildStream installation instructions <main_install>` for the different options to install them