Specs: make element registrations be per-module, define how they are

sky/specs/modules.md

Index: sky/specs/modules.md
diff --git a/sky/specs/modules.md b/sky/specs/modules.md
index 50fae43ae769a86dcfd4773831c416651089efc7..86a1d1ff9c403352b202a5174c78fb0b3de3f5bc 100644
--- a/sky/specs/modules.md
+++ b/sky/specs/modules.md
@@ -24,6 +24,7 @@ finished parsing, the document waits until its list of outstanding
 dependencies is empty before the module it represents is marked
 complete.
 
+
 Module API
 ----------
 
@@ -47,6 +48,14 @@ its ```Module``` object. By default, the ```exports``` property of a
 import is still useful (it exposes its contents, e.g. templates that
 the import might have been written to provide).
 
+### Exporting element definitions ###
+
+When importing a module into another, Sky looks at the properties on
+the imported module's ``exports`` value, and for each property that is
+an element constructor (generated by ``registerElement()``), it adds
+an element to the importee's element registry.
+
+ 
 Naming modules
 --------------