Add rationale to ttx_roundtrip check

com.google.fonts/check/ttx_roundtrip
On the Universal profile.

(PR #4570)

CHANGELOG.md

@@ -21,6 +21,7 @@ A more detailed list of changes is available in the corresponding milestones for
 
 #### On the Universal profile
   - **DISABLED - [com.google.fonts/check/legacy_accents]:** This is one of the checks that we probably should run on the sources instead of binaries. (https://github.com/fonttools/fontbakery/issues/3959#issuecomment-1822913547)
+  - **[com.google.fonts/check/ttx_roundtrip]:** Added rationale. (PR #4570)
 
 #### On the Open Type Profile
   - **[com.google.fonts/check/layout_valid_feature_tags]:** Updated the check to allow valid private-use feature tags. (issue #4544)

Lib/fontbakery/checks/universal/__init__.py

@@ -1296,6 +1296,13 @@ def com_google_fonts_check_unique_glyphnames(ttFont):
 @check(
     id="com.google.fonts/check/ttx_roundtrip",
     conditions=["not vtt_talk_sources"],
+    rationale="""
+        One way of testing whether or not fonts are well-formed at the
+        binary level is to convert them to TTX and then back to binary. Structural
+        problems within the binary font will show up as errors during conversion.
+        This is not necessarily something that a designer will be able to address
+        but is evidence of a potential bug in the font compiler used to generate
+        the binary.""",
     proposal="https://github.com/fonttools/fontbakery/issues/1763",
 )
 def com_google_fonts_check_ttx_roundtrip(font):