Add extract usage to readme

README.md

@@ -77,6 +77,18 @@ warden schema validate schema.json
 find _warden -name \*.warden -print0 | xargs -0 warden marker failed
 ```
 
+Extracting numeric samples
+--------------------------
+
+Samples can be extracted as CSV files from view markers for easier
+interactive analysis.
+
+```
+warden-sample extract \
+    -o samples.csv \
+    _warden/$view/$data_dates/*/*.json
+```
+
 Future Work
 -----------
 
@@ -91,4 +103,4 @@ Further reading
  - [Documentation](https://github.com/ambiata/warden/tree/master/doc)
    (or find a built version on S3 under
    `s3://ambiata-dispensary-v2/doc/master/warden/`).
- - [Talk slides](https://github.com/ambiata/talks/blob/master/warden-2016-05-20/slides.pdf).
+ - [Talk slides](https://github.com/ambiata/talks/blob/master/warden-2016-05-20/slides.pdf).