add more info to readme

Author: wysiwys

README.md

@@ -1,11 +1,14 @@
 # GitHub Action for extraction and transformation of benchmark data
 
+GitHub Action to transform benchmark output into a standardized format, and write to a file.
+
 Currently supported sources:
  - `cargo bench`
  - `criterion`
 
 ## Fields
 - `name` (required): name of the benchmark
+- `tool` (required): tool used to get benchmark output. One of `["cargo"]`
 - `platform` (required): a string describing the platform
-- `output-file-path`: a path to a file containing the benchmark output
-- `data-out-path`: the path where the output of the action should be written
+- `output-file-path` (required): a path to a file containing the output of the benchmark tool
+- `data-out-path` (required): the path where the output of the action should be written