Document usage in the README

README.md

@@ -15,7 +15,75 @@ All Zika-specific steps and functionality for the Nextstrain pipeline should be
 housed in this repository.
 
 
+## Usage
+
+The easiest way to run this pathogen build is using the [Nextstrain
+command-line tool][nextstrain-cli]:
+
+    nextstrain build .
+
+See the [nextstrain-cli README][] for how to install the `nextstrain` command.
+
+Alternatively, you should be able to run the build using `snakemake` within an
+suitably-configured local environment.  Details of setting that up are not yet
+well-documented, but will be in the future.
+
+All build output files will go into a new `build/` directory.
+
+Once you've run the build, you can view the results in auspice:
+
+    nextstrain view build/auspice/
+
+
+## Configuration
+
+The main configuration file is `config.yaml`, with supporting files in the
+`config/` directory.
+
+To override any part of the config, you can create a file named
+`config_local.yaml` and add the bits you want you change.  For example, it's
+useful during development to speed up build times by very heavily subsampling
+the dataset.  You can use this `config_local.yaml` to do that:
+
+```yaml
+---
+filter:
+    sequences_per_category: 1
+```
+
+### fauna / RethinkDB credentials
+
+This build starts by pulling sequences from our live [fauna][] database (a
+RethinkDB instance).  For data privacy and security reasons, you'll need to
+provide credentials to the database yourself, using a `config_local.yaml`
+snippet like so:
+
+```yaml
+---
+credentials:
+  rethink:
+    host: ...
+    auth_key: ...
+```
+
+If you don't have access to our database, you can run the build using the
+example data provided in this repository.  Before running the build, copy the
+sequences into the `build/` directory like so:
+
+    mkdir -p build/data/
+    cp example_data/zika.fasta build/data/
+
+### Per-run config
+
+It is also possible to override config values for each run by passing
+`--config` or `--configfile` options to `snakemake`.  See the [`snakemake`
+command-line options documentation][snakemake cli].
+
+
 [Nextstrain]: https://nextstrain.org
 [fauna]: https://github.com/nextstrain/fauna
 [augur]: https://github.com/nextstrain/augur
 [auspice]: https://github.com/nextstrain/auspice
+[snakemake cli]: https://snakemake.readthedocs.io/en/stable/executable.html#all-options
+[nextstrain-cli]: https://github.com/nextstrain/cli
+[nextstrain-cli README]: https://github.com/nextstrain/cli/blob/master/README.md