Add function 'digest' doc

Add function 'digest' guide doc in hot_spec.rst.

Change-Id: Ia027a483c63f748437778d602eb04524a7489ea5

doc/source/template_guide/hot_spec.rst

@@ -153,6 +153,7 @@ For example, Heat currently supports the following values for the
   get_resource
   list_join
   repeat
+  digest
   resource_facade
   str_replace
   Fn::Select
@@ -818,6 +819,40 @@ A sample use of this function with a simple list is shown below.
 This would resolve to "one, two, and three".
 
 
+digest
+--------
+The *digest* function allows for performing digest operations on a given value.
+This function has been introduced in the Kilo release and is usable with HOT versions
+later than `2015-04-30`.
+
+The syntax of the digest function is as follows:
+
+::
+
+  digest:
+    - <algorithm>
+    - <value>
+
+algorithm
+    The digest algorithm. Valid algorithms are the ones
+    provided natively by hashlib (md5, sha1, sha224, sha256, sha384,
+    and sha512) or any one provided by OpenSSL.
+value
+    The value to digest. This function will resolve to the corresponding hash of
+    the value.
+
+
+An example of how to use the digest function is shown below:
+
+::
+
+  # from a user supplied parameter
+  pwd_hash: { digest: ['sha512', { get_param: raw_password }] }
+
+The value of the digest function would resolve to the corresponding hash of
+the value of 'raw_password'.
+
+
 repeat
 ------
 The *repeat* function allows for dynamically transforming lists by iterating