Add export-glyph-names to sample.toml (#1403)

and also add a reference to the full docs

private-build-plans.sample.toml

@@ -1,7 +1,11 @@
+### See https://github.com/be5invis/Iosevka/blob/master/doc/custom-build.md for comprehensive 
+### documentation of the options in this file
+
 [buildPlans.iosevka-custom]      # <iosevka-custom> is your plan name
 family = "Iosevka Custom"        # Font menu family name
 spacing = "normal"               # Optional; Values: `normal`, `term`, `fontconfig-mono`, or `fixed`
 serifs = "sans"                  # Optional; Values: `sans` or `slab`
+export-glyph-names = false       # Set this to true for ligature support in Kitty (increased file size)
 
 webfont-formats = ['ttf', 'woff2']