Add SOCIAL_AUTH_IMMUTABLE_USER_FIELDS to docs (#84)

* Add new SOCIAL_AUTH_IMMUTABLE_USER_FIELDS setting to docs
* Minor tweak to formatting

docs/configuration/settings.rst

@@ -279,6 +279,16 @@ Miscellaneous settings
     ``email``, etc.), those fields need to be *protect*. Set any field name that
     requires *protection* in this setting, and it won't be updated.
 
+
+``SOCIAL_AUTH_IMMUTABLE_USER_FIELDS = ['email',]``
+    Set any field name that requires *protection* in this setting, and it won't
+    be updated after inital population. This setting is similar to 
+    ``SOCIAL_AUTH_PROTECTED_USER_FIELDS`` in that they both do not allow changes 
+    of the data - however this one allows it to be set if no prior value exists.
+    An example use case might be an application that seeds data from a social 
+    plaform but allows the users to override it locally.    
+
+
 ``SOCIAL_AUTH_SESSION_EXPIRATION = False``
     By default, user session expiration time will be set by your web
     framework (in Django, for example, it is set with