update changelog with note about most_common_width bug fix
Below are the most important changes from each release.
A more detailed list of changes is available in the corresponding milestones for each release in the Github issue tracker (https://github.com/googlefonts/fontbakery/milestones?state=closed).
+## 0.6.13 (2019-??-??)
+### Bug fixes
+ - Fix bug in handling of `most_common_width` in `glyph_metrics_stats` which affected checking of monospaced metadata. (PR #2391)
## 0.6.12 (2019-Mar-11)
### Bug fixes
