rename setupDSIG to setupDummyDSIG and added a doc string to clarify the legacy status of this

Lib/fontTools/fontBuilder.py

@@ -498,7 +498,10 @@ class FontBuilder(object):
             defaults = _maxpDefaultsOTF
         self._initTableWithValues("maxp", defaults, {})
 
-    def setupDSIG(self):
+    def setupDummyDSIG(self):
+        """This adds a dummy DSIG table to the font to make some MS applications
+        happy. This does not properly sign the font.
+        """
         from .ttLib.tables.D_S_I_G_ import SignatureRecord
 
         sig = SignatureRecord()

Tests/fontBuilder/fontBuilder_test.py

@@ -70,7 +70,7 @@ def test_build_ttf(tmpdir):
     fb.setupNameTable(nameStrings)
     fb.setupOS2()
     fb.setupPost()
-    fb.setupDSIG()
+    fb.setupDummyDSIG()
 
     fb.save(outPath)
 
@@ -101,7 +101,7 @@ def test_build_otf(tmpdir):
     fb.setupNameTable(nameStrings)
     fb.setupOS2()
     fb.setupPost()
-    fb.setupDSIG()
+    fb.setupDummyDSIG()
 
     fb.save(outPath)