[cantarell-fonts] Readme: Mention build options

[Nikolaus Waxweiler <nwaxweiler src gnome org>]

commit 7c454c4add7da6dda314cd37362d1d73d5a1b6ee
Author: Nikolaus Waxweiler <madigens gmail com>
Date:   Fri Aug 27 21:07:09 2021 +0100

    Readme: Mention build options

 README.md | 2 ++
 1 file changed, 2 insertions(+)
---
diff --git a/README.md b/README.md
index 99687226..201b14e4 100644
--- a/README.md
+++ b/README.md
@@ -18,6 +18,8 @@ meson build
 ninja -C build install
 ```
 
+By default, only the variable font is built. You can toggle either with the `buildstatics` and `buildvf` 
Meson option.
+
 Contributing
 ------------