Add a doctest

Author: kwankyu

src/sage/modules/submodule.py

@@ -98,6 +98,17 @@ def __init__(self, ambient, gens, check=True, already_echelonized=False):
             sage: N = M.submodule([vector([x - y, z]), vector([y * z, x * z])])
             sage: N.is_submodule(M)
             True
+
+        ::
+
+            sage: R.<x,y> = QQ[]
+            sage: S = R.quotient([x*y])
+            sage: M = FreeModule(S, 2)
+            sage: M.zero_submodule()
+            Submodule of Ambient free module of rank 2 over Quotient of Multivariate Polynomial Ring in x, y
+             over Rational Field by the ideal (x*y)
+              Generated by the rows of the matrix:
+              []
         """
         if not isinstance(ambient, (FreeModule_ambient, QuotientModule_free_ambient)):
             raise TypeError("ambient (=%s) must be ambient or a quotient" % ambient)