fix broken links (#26)

* rely on auto-linking of base URLs

README.md

@@ -50,13 +50,12 @@ rule should be able to be reasoned with on its own.
 ## Input data
 
 This build starts by downloading sequences from
-[https://data.nextstrain.org/files/zika/sequences.fasta.xz](data.nextstrain.org/files/zika/sequences.fasta.xz)
+https://data.nextstrain.org/files/zika/sequences.fasta.xz
 and metadata from
-[https://data.nextstrain.org/files/zika/metadata.tsv.gz](data.nextstrain.org/files/zika/metadata.tsv.gz).
+https://data.nextstrain.org/files/zika/metadata.tsv.gz.
 These are publicly provisioned data by the Nextstrain team by pulling sequences
-from NCBI GenBank via ViPR and performing additional bespoke curation. Our
-curation is described
-[here](https://github.com/nextstrain/fauna/blob/master/builds/ZIKA.md).
+from NCBI GenBank via ViPR and performing 
+[additional bespoke curation](https://github.com/nextstrain/fauna/blob/master/builds/ZIKA.md).
 
 Data from GenBank follows Open Data principles, such that we can make input data
 and intermediate files available for further analysis. Open Data is data that