Specs: write up what we've discussed about fonts, just so that it's written down somewhere (this is…

sky/specs/fonts.md

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+Fonts
+=====
+
+Mojo has a font service, which takes a URL and hands back glyphs.
+
+Sky has an API that takes a URL, a name, and hands back a promise
+which, when resolved, indicates that Sky has now associated that font
+name with the relevant glyphs (by calling the Mojo service).
+
+The text part of Sky's drawing API accepts a list of font names, and
+uses those to draw the relevant glyphs, falling back through the
+provided fonts, then all the loaded fonts, until a glyph is found.
+
+By default, three fonts are loaded, with the names 'serif',
+'sans-serif', and 'monospace'. They have good Unicode coverage.
+
+TODO(ianh): Actually define these APIs and so on.