docs(contributing): add performance label.

The label got added to the tracker, so I'm adding a description of when it's used. :)

CONTRIBUTING.md

@@ -178,6 +178,7 @@ Label | Apply When | Notes
 `enhancement` | Added to [feature requests](#request-a-feature), PRs, or documentation issues that are purely additive: the code or docs currently work as expected, but a change is being requested or suggested. |
 `help wanted` | Applied by [Committers](#join-the-project-team) to issues and PRs that they would like to get outside help for. Generally, this means it's lower priority for the maintainer team to itself implement, but that the community is encouraged to pick up if they so desire | Never applied on first-pass labeling.
 `in-progress` | Applied by [Committers](#join-the-project-team) to PRs that are pending some work before they're ready for review. | The original PR submitter should @mention the team member that applied the label once the PR is complete.
+`performance` | This issue or PR is directly related to improving performance. |
 `starter` | Applied by [Committers](#join-the-project-team) to issues that they consider good introductions to the project for people who have not contributed before. These are not necessarily "easy", but rather focused around how much context is necessary in order to understand what needs to be done for this project in particular. | Existing project members are expected to stay away from these unless they increase in priority.
 `support` | This issue is either asking a question about how to use the project, clarifying the reason for unexpected behavior, or possibly reporting a `bug` but does not have enough detail yet to determine whether it would count as such. | The label should be switched to `bug` if reliable reproduction steps are provided. Issues primarily with unintended configurations of a user's environment are not considered bugs, even if they cause crashes.
 `tests` | This issue or PR either requests or adds primarily tests to the project. | If a PR is pending tests, that will be handled through the [PR review process](#review-pull-requests)