mbrola: add mb-nl1 and mb-nl3 voices

docs/mbrola.md

 | `ma1`        | Malay                | female | `mb-ma1` |                |
 | `mx1`        | Mexican Spanish      | male   | `mb-mx1` | `mbrola-mx1`   |
 | `mx2`        | Mexican Spanish      | male   | `mb-mx2` | `mbrola-mx2`   |
-| `nl1`        | Dutch                | male   |          |                |
+| `nl1`        | Dutch                | male   | `mb-nl1` <sup>[1](#footnote1)</sup> |   |
 | `nl2`        | Dutch                | male   | `mb-nl2` | `mbrola-nl2`   |
-| `nl3`        | Dutch                | female |          |                |
+| `nl3`        | Dutch                | female | `mb-nl3` |                |
 | `nz1`        | Maori                | male   |          |                |
 | `pl1`        | Polish               | female | `mb-pl1` | `mbrola-pl1`   |
 | `pt1`        | Portuguese           | female | `mb-pt1` | `mbrola-pt1`   |
 When `Makefile.am` is update, when espek-ng will be compiled, e.g. by
 entering command `automake; make -B` newly added MBROLA voice will be compiled automatically.
 
+<a name="footnote1">1</a>: This voice has very limited set of diphones and is usable only for reading numbers.
+

espeak-ng-data/voices/mb/mb-nl1

+name dutch-mbrola-1
+language nl 9
+gender male
+mbrola nl1 nl_phtrans
+

espeak-ng-data/voices/mb/mb-nl3

+name dutch-mbrola-3
+language nl 9
+gender female
+pitch 130 200
+mbrola nl3 nl_phtrans
+