Add example for --subsample-max-sequences

src/guides/bioinformatics/filtering-and-subsampling.rst

@@ -69,6 +69,23 @@ sequence per month from each country:
      --output-sequences subsampled_sequences.fasta \
      --output-metadata subsampled_metadata.tsv
 
+An alternative to ``--sequences-per-group`` is ``--subsample-max-sequences``.
+This is useful if you don't know how many groups the metadata will be
+partitioned into but you have a target sample size. For example, target 100
+total sequences:
+
+.. code-block:: bash
+
+   augur filter \
+     --sequences data/sequences.fasta \
+     --metadata data/metadata.tsv \
+     --min-date 2012 \
+     --exclude exclude.txt \
+     --group-by country year month \
+     --subsample-max-sequences 100 \
+     --output-sequences subsampled_sequences.fasta \
+     --output-metadata subsampled_metadata.tsv
+
 Subsampling using multiple ``augur filter`` commands
 ====================================================