added a doc for inheritable namespace

diff --git a/doc/build/content/namespaces.txt b/doc/build/content/namespaces.txt
index bdaba9b..fe90a5a 100644
--- a/doc/build/content/namespaces.txt
+++ b/doc/build/content/namespaces.txt

@@ -114,3 +114,18 @@
 
 The `self` namespace, in the case of a template that does not use inheritance, is synonomous with `local`.  If inheritance is used, then `self` references the topmost template in the inheritance chain, where it is most useful for providing the ultimate form of various "method" calls which may have been overridden at various points in an inheritance chain.  See [inheritance](rel:inheritance).
 
+### Inheritable Namespaces
+
+The `<%namespace>` tag includes an optional attribute `inheritable="True"`, which will cause the namespace to be attached to the `self` namespace.  Since `self` is globally available throughout an inheritance chain (described in the next section), all the templates in an inheritance chain can get at the namespace via self.
+
+    ## base.html
+    <%namespace name="foo" file="foo.html" inheritable="True"/>
+    
+    ${next.body()}
+
+    ## somefile.html
+    <%import file="base.html"/>
+    
+    ${self.foo.bar()}
+    
+The `import="*"` part of the `<%namespace>` tag doesn't yet interact with the `inheritable` flag, more on that in a future release.
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