Adds font instructions.

README.md

 # onlyoffice
 
+## Fonts
+
+If you want to add fonts, place the font folder in ttf, and then add a file called `docker-compose.override.yml`:
+```
+version: '2'
+services:
+  onlyoffice:
+    volumes:
+    - ./ttf/Cairo:/usr/share/fonts/truetype/Cairo
+    - ./ttf/Roboto_Slab:/usr/share/fonts/truetype/Roboto_Slab
+```
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