Meta: mention type:untestable and type:missing-coverage wpt labels (#1211)

Author: foolip

CONTRIBUTING.md

@@ -10,7 +10,7 @@ Updates to the spec must reference [resolved issues marked `needs spec`](https:/
 
 To make edits to the design, please send pull requests against the Nightly spec on the master branch. We use [bikeshed](https://github.com/tabatkins/bikeshed). So, change `docs/index.bs` and submit it with the corresponding bikesheded `index.html`.
 
-For normative changes, a corresponding [web-platform-tests](https://github.com/w3c/web-platform-tests) PR is highly appreciated. Typically, both PRs will be merged at the same time. Note that a test change that contradicts the spec should not be merged before the corresponding spec change. If testing is not practical, please explain why and if appropriate [file an issue](https://github.com/w3c/web-platform-tests/issues/new) to follow up later.
+For normative changes, a corresponding [web-platform-tests](https://github.com/w3c/web-platform-tests) PR is highly appreciated. Typically, both PRs will be merged at the same time. Note that a test change that contradicts the spec should not be merged before the corresponding spec change. If testing is not practical, please explain why and if appropriate [file an issue](https://github.com/w3c/web-platform-tests/issues/new) to follow up later. Add the `type:untestable` or `type:missing-coverage` label as appropriate.
 
 Contributions to this repository are intended to become part of Recommendation-track documents
 governed by the [W3C Patent Policy](http://www.w3.org/Consortium/Patent-Policy-20040205/) and