doc: add license URLs so go's doc service can detect our license

Author: Stebalien

LICENSE

@@ -3,3 +3,6 @@ Unless otherwise noted, all code contributed prior to 2019-05-06 and not contrib
 a user listed in [this signoff issue](https://github.com/ipfs/go-ipfs/issues/6302) is 
 licensed under MIT-only. All new contributions (and past contributions since 2019-05-06) 
 are licensed under a dual MIT/Apache-2.0 license.
+
+MIT: https://www.opensource.org/licenses/mit
+Apache-2.0: https://www.apache.org/licenses/license-2.0