update docs

README.md

@@ -76,7 +76,14 @@ Operations are transformations to perform on the inputs.
 An operation can produce one or more new assets, which can then be referenced in
 later operations.
 
-The operations that JVCL either supports or intends to support are:
+Most of the operation settings can be JavaScript expressions, for example:
+
+    "startTime": "someAsset.duration - 10"
+
+The above would set the `startTime` value to ten seconds less than the duration of `someAsset`.
+
+### Supported Operations
+Today, JVCL supports these operations:
 
 ### split
 Split an audio/video asset into multiple assets