added noupdates proposal

doc/proposals/noupdates.txt

+
+Automatic updates
+=================
+
+The Module version of RandomStreams could arrange for the automatic update of
+certain inputs (such as the random number generators) at the time of make(), so
+that certain *obvious* patterns would work:
+
+>>> rs = RandomStreams()
+>>> u = rs.uniform(...)
+>>> f = theano.function([], u)
+>>> assert not numpy.all(f() == f())
+
+Unfortunately, with shared variables this does not work!  Function needs to be
+told which shared variables to update.  The current workaround is to do this:
+
+>>> theano.function([], u, updates=rs.updates())
+
+or this:
+
+>>> theano.function([], u, updates=[u.update])
+
+But it is all too easy to forget to do either of these workarounds, and
+accidentally run a program whose random numbers are the same in every call.
+
+
+Proposal
+========
+
+Add an optional `default_update` attribute to Shared variables. This will be
+consulted by function.  If no update expression is given for this variable in
+the updates list, then this default will be inserted.  Note well: a value of None for the
+default_update means to update with a value of None!  To have no default update,
+make sure that the default_update attribute is not defined.
+
+Add an optional argument to function: `no_default_updates`.  This argument defaults to
+False, which results in the current semantics.
+A True value here would mean "ignore all default_update expressions", and this
+would be useful for disabling implicit behaviour.
+A list of shared variables here would mean to ignore the
+default_update_expressions in these specific variables.
+
+
+Alternatives
+============
+
+Consider a singleton 'NOUPDATE' object that can be used as a pseudo-expression
+in the update list.  This doesn't introduce a new keyword argument, which makes
+it slightly more awkward to document in theano.function.  Really though, I have
+no strong feelings between this and the no_updates paramter.
+