Added alternative sources repo for font files

Author: rougier

README.md

@@ -17,12 +17,21 @@
 1. You need to create a `fonts` repository with:
 
 * `fonts/roboto/*`           : See https://fonts.google.com/specimen/Roboto
+                                or https://github.com/googlefonts/roboto/tree/master/src/hinted
 * `fonts/roboto-slab/*`      : See https://fonts.google.com/specimen/Roboto+Slab
+                                or https://github.com/googlefonts/robotoslab/tree/master/fonts/static
 * `fonts/source-code-pro/*`  : See https://fonts.google.com/specimen/Source+Code+Pro
+                                or https://github.com/adobe-fonts/source-code-pro/tree/release/OTF
 * `fonts/source-sans-pro/*`  : See https://fonts.google.com/specimen/Source+Sans+Pro
+                                or https://github.com/adobe-fonts/source-sans-pro/tree/release/OTF
 * `fonts/source-serif-pro/*` : See https://fonts.google.com/specimen/Source+Serif+Pro
+                                or https://github.com/adobe-fonts/source-serif-pro/tree/release/OTF
 * `fonts/delicious-123/*`    : See https://www.exljbris.com/delicious.html
 
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 2. You need to generate all the figures:
 
 ```