Update Spacebars REAMDE: mention nested sub-expressions

packages/spacebars/README.md

@@ -495,6 +495,25 @@ We can write {{foo}} and it doesn't matter.
 
 Comment tags can be used wherever other template tags are allowed.
 
+## Nested sub-expressions
+
+Sometimes an argument to a helper call is best expressed as a return value of
+some other expression. For this and other cases, one can use parentheses to
+express the evaluation order of nested expressions.
+
+```handlebars
+{{capitalize (getSummary post)}}
+```
+
+In this example, the result of the `getSummary` helper call will be passed to
+the `capitalize` helper.
+
+Sub-expressions can be used to calculate key-word arguments, too:
+
+```handlebars
+{{> tmpl arg=(helper post)}}
+```
+
 ## HTML Dialect
 
 Spacebars templates are written in [standard