Add Naming section with naming and namespacing

notes/style.md

@@ -270,6 +270,38 @@ possible inherited variables right in the short description, using
 naive Bash notation (no quoting) as in above example.
 
 
+Naming
+------
+
+### Module name and namespacing ###
+
+A valid Shellfu module name consists of lowercase letters, numbers and
+underscore, ie. `[a-z_][a-z0-9_]`.  This name must be used as name of
+the module file, without the extension.
+
+Module must not define a global variable or function that does not start
+with its name.  That is, a global variable or function may be:
+
+ *  Just module name itself.
+
+ *  Module name itself followed by *two underscores** (`__`) and
+    an id consisting of one or more letters, numbers or underscores.
+
+ *  Any of above, prefixed by one or more underscores.
+
+
+### Private names ###
+
+Names starting with **double underscore** are considered strongly private:
+that is, they must only be used in the same file.
+
+Names starting with **single underscore** are considered weakly private: they
+may be used in other modules within the same project.  (This can be useful
+in plugin scenarios.)
+
+When in doubt, use **double underscore**.
+
+
 Common language constructs
 --------------------------