add back value must be a number in --measures

src/commands/measure/README.md

@@ -19,6 +19,7 @@ datadog-ci measure --level job --measures binary.size:1024
   span for the currently running job.
 - `--no-fail` (default: `false`) will prevent the measure command from failing if there are issues submitting the data.
 - `--measures` is an array of key value pairs of the shape `key:value`. This will be the measures added to the pipeline or job span.
+  The `value` must be a number
 - `--measures-file` is a path to a file containing the measures to be added to the pipeline or job span. The file should be a JSON file with the following structure:
   ```json
   {