Ignore path starting with _ on compose_node_name

https://github.com/salt-formulas/reclass/pull/48#issuecomment-406823623
diff --git a/README-extentions.rst b/README-extentions.rst
index 2053a2d..2bc4816 100644
--- a/README-extentions.rst
+++ b/README-extentions.rst
@@ -536,3 +536,5 @@
       ...
     staging.mysql:
       ...
+
+If the subfolder path starts with the underscore character ``_``, then the subfolder path is NOT added to the node name.