Add a comment explaining what compatcheck does

diff --git a/compatcheck b/compatcheck
index a11478f36e9c9700b07e8edd700753a1968a618a..fc86ac1fb04262bd11d594aa1a57941718202a97 100755 (executable)
--- a/compatcheck
+++ b/compatcheck
@@ -2,6 +2,24 @@
 
 # SPDX-License-Identifier: CC0-1.0
 
+# This is a script to check the backward compatibility of fonts, and
+# specifically of Bedstead.  It takes two font files as arguments and
+# checks whether upgrades from the first to the second will break
+# anything.  It should fail on anything that would require a change of
+# major version number.
+#
+# Things that are checked:
+#
+# Glyph names.  Existing PostScript files that refer to specific glyph
+# names should continue to work.  The script checks that all glyph
+# names in the old font still exist, but not that they look right.
+#
+# Code points.  Anything using a font through the 'cmap' table should
+# continue to work.  That means that any 'cmap' that exists should
+# continue to exist, and that any mapping through any 'cmap' should
+# continue to work.  As with glyph names, the script checks for the
+# existence of mappings, but not their content.
+
 from argparse import ArgumentParser
 from fontTools import ttLib
 from sys import exit