doc: update readme for coverage changes

Mention that `crate_features` is now optional, and also update a very
outdated reference to `cargo kcov`

Signed-off-by: Patrick Roy <[email protected]>

Author: roypat

README.md

@@ -44,10 +44,13 @@ The json must have the following fields:
   if multiple files are to be skipped, they must be separated with `|`).
   It should be used to exclude autogenerated files. If the repository does not
   have any autogenerated files, `exclude_path` should be an empty string.
-- `crate_features`: `cargo kcov` does not build crate features by default. To
-  get the coverage report including optional features, these need to be
-  specified in `crate_features` separated by comma. If the crate does not have
-  any features, this field should be empty.
+
+Additionally, the following optional fields are available:
+
+- `crate_features`: By default, we pass `--all-features` to cargo when collecting
+  coverage. To have the coverage report only include a subset of features (in addition 
+  to default features, which are always enabled), specify them in this field
+  as a comma-separated string.
 
 This file is required for the coverage integration so it needs to be added
 to the repository as well.