[readme] update language around provisioning data files
@@ -45,7 +45,12 @@ Or get in touch over email at hello@nextstrain.org or on Twitter at [@nextstrain
## Building
-After provisioning the `data/sequences.fasta` file, the entire build can be regenerated by running
+In order to run the nextstrain build you must provision `data/sequences.fasta` and `data/metadata.tsv`.
+Due to data sharing agreements, we are not able to publicaly provide these files (please see [this issue](https://github.com/nextstrain/ncov/issues/53) as we are trying to improve this).
+
+
+After this, the entire build can be regenerated by running
+
```
snakemake -p
```
