update change log with 0.7.3 section & find_empty_letters check
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).
+## 0.7.3 (2019-Apr-15)
+
+### New checks
+ - **[com.adobe.fonts/check/find_empty_letters]:** Letters in font have glyphs that are not empty? (PR #2460)
+
## 0.7.2 (2019-Apr-09)
### Note-worthy code changes
- **[com.google.fonts/check/name/family_and_style_max_length]:** increased max length to 27 chars. After discussing the problem in more detail at issue #2179 we decided that allowing up to 27 chars would still be on the safe side. Please also see issue #2447
