Add NAMESERVER_TTL and associated documentation. (#398)

docs/_functions/domain/NAMESERVER_TTL.md

@@ -0,0 +1,19 @@
+---
+name: NAMESERVER_TTL
+parameters:
+  - ttl
+---
+
+TTL sets the TTL on the domain apex NS RRs defined by [NAMESERVER](#NAMESERVER).
+
+The value can be an integer or a string. See [TTL](#TTL) for examples.
+
+{% include startExample.html %}
+{% highlight js %}
+
+D('example.com', REGISTRAR, DnsProvider('R53'),
+  NAMESERVER_TTL('2d'),
+  NAMESERVER('ns')
+);
+{%endhighlight%}
+{% include endExample.html %}