Update gui_using_fonts.rst

Add link to *open-source* alternative of BMFont editor

(cherry picked from commit 6c7b0917bbd3822fcd4d17f14937081158de75be)

tutorials/ui/gui_using_fonts.rst

@@ -66,7 +66,7 @@ Bitmap fonts
 
 Godot supports the BMFont (``.fnt``) bitmap font format. This is a format created
 by the `BMFont <https://www.angelcode.com/products/bmfont/>`__ program. Many
-BMFont-compatible programs also exist, like `BMGlyph <https://www.bmglyph.com/>`__.
+BMFont-compatible programs also exist, like `BMGlyph <https://www.bmglyph.com/>`__ or web-based `fontcutter <https://github.com/fabienbk/fontcutter>`__.
 
 Alternatively, you can import any image to be used as a bitmap font. This is
 only supported for **monospaced** fonts (fonts where each character has the same