[cantarell-fonts] Readme: Mention glyphsLib minimal version

[Nikolaus Waxweiler <nwaxweiler src gnome org>]

commit 53454dfb62bc69ba2917222ed25a39782051ee76
Author: Nikolaus Waxweiler <madigens gmail com>
Date:   Fri Aug 10 21:30:34 2018 +0100

    Readme: Mention glyphsLib minimal version

 README.md | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)
---
diff --git a/README.md b/README.md
index 42f227ee..9ebfdc84 100644
--- a/README.md
+++ b/README.md
@@ -12,7 +12,7 @@ python3 -m venv venv
 venv/Scripts/activate  # ...or on Windows cmd.exe or PowerShell
 
 pip3 install meson ninja  # Unless already present on system.
-pip3 install fontmake psautohint
+pip3 install fontmake psautohint "glyphslib>=3.0.3"
 
 meson build
 ninja -C build install