stack/supervisord: add how to use documentation

stack/supervisord/README.rst

+Supervisord process manager
+
+How to use
+==========
+
+Supervisord stack provides a library which can he called in your instance slapos. This stack can be used to run sub services in a partition.
+
+To use:
+
+ * extend ``stack/supervisord/buildout.cfg`` in your software.cfg file.
+ * provide ``supervisord-library:target`` and ``supervisord-conf:target`` to your instance template which require to use supervisord controller.
+ * add ``{% import "supervisord_lib" as supervisord_lib with context %}`` to instance template which call supervisord library. See example bellow:
+
+**software.cfg**
+::
+
+  [template-instance]
+  recipe = slapos.recipe.template:jinja2
+  context =
+      key buildout_bin_directory buildout:bin-directory
+      key supervisord        supervisord-library:target
+      key supervisord_conf   supervisord-conf:target
+
+**instance.cfg.in**
+::
+
+    [template-custom-instance.cfg]
+    recipe  = slapos.recipe.template:jinja2
+    supervisord-lib = {{ supervisord }}
+    import-list =
+      file supervisord :supervisord-lib
+    context =
+        raw buildout_bin_directory  {{ buildout_bin_directory }}
+        raw supervisord_conf        {{ supervisord_conf }}
+
+
+**custom-instance.cfg**
+::
+
+  {% import "supervisord" as supervisord with context %}
+  {{ supervisord.supervisord("custom-controller", buildout_bin_directory, supervisord_conf, use_service_hash=False) }}
+
+  # add program to service controller
+  {% set program_dict = {"name": "mariadb", "command": "${mariadb-service:wrapper}",
+    "stopwaitsecs": 300, "environment": []} %}
+
+  {{ supervisord.supervisord_program("custom-controller", "mariadb", program_dict) }}
+
+  ...
+  
+  [buildout]
+  
+  parts = 
+    ...
+    custom-controller-service
+    custom-controller-mariadb
+