Created Yggdrasil-only router configuration (markdown)

Yggdrasil-only-router-configuration.md

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+For a router talking to other routers and reseed servers through Yggdrasil only.  
+Start yggdrasil first. 
+Create ~/.i2pd/i2pd.conf
+
+```ini
+daemon=true
+ipv4=false
+ipv6=false
+ssu=false
+ntcp2.enabled=false
+meshnets.yggdrasil=true  
+```
+
+If you use Yggdrasil subnet and want to bind your router to a particular address specify it with  
+meshnets.yggaddress= your local yggdrasil address  
+
+Start i2pd, it will also reseed from an yggdrasil reseed server. 
+No clearnet communication is involved.