Update CONTRIBUTING.md

Author: Mec-iS

.github/CONTRIBUTING.md

@@ -22,7 +22,7 @@ Please note we have a [code of conduct](CODE_OF_CONDUCT.md), please follow it in
 
 1. After a PR is opened maintainers are notified
 2. Probably changes will be required to comply with the workflow, these commands are run automatically and all tests shall pass:
-    * **Coverage**: `tarpaulin` is used with command `cargo tarpaulin --out Lcov --all-features -- --test-threads 1`
+    * **Coverage** (optional): `tarpaulin` is used with command `cargo tarpaulin --out Lcov --all-features -- --test-threads 1`
     * **Linting**: `clippy` is used with command `cargo clippy --all-features -- -Drust-2018-idioms -Dwarnings`
     * **Testing**: multiple test pipelines are run for different targets
 3. When everything is OK, code is merged.
@@ -40,4 +40,4 @@ Please note we have a [code of conduct](CODE_OF_CONDUCT.md), please follow it in
 
 * **PRs on develop**: any change should be PRed first in `development`
 
-* **testing**:  everything should work and be tested as defined in the workflow. If any is failing for non-related reasons, annotate the test failure in the PR comment.
+* **testing**:  everything should work and be tested as defined in the workflow. If any is failing for non-related reasons, annotate the test failure in the PR comment.