Add documentation for review request handling feature

Author: WaffleLapkin

src/SUMMARY.md

@@ -28,6 +28,7 @@
     - [Pinging](./triagebot/pinging.md)
     - [Requesting Prioritization](./triagebot/requesting-prioritization.md)
     - [Review Changes Requested](./triagebot/review-submitted.md)
+    - [Review Requested](./triagebot/review-requested.md)
     - [Rustc Commit Tracking](./triagebot/rustc-commit-list.md)
     - [Shortcuts](./triagebot/shortcuts.md)
     - [Triagebot Dashboard](./triagebot/triage-dashboard.md)

src/triagebot/review-requested.md

@@ -0,0 +1,24 @@
+# Review Requested
+
+This feature will automatically adjust the labels on a pull request when the PR author requests a review from an assignee.
+
+## Usage
+
+In the list of reviewers, click the "Re-request review" button near an assignee's name.
+This will automatically remove the "waiting on the author" labels, and add a new labels to indicate that the PR is waiting on the review.
+
+## Configuration
+
+This feature is enabled on a repository by having a `[review-requested]` table in `triagebot.toml`:
+
+```toml
+[review-requested]
+# Those labels are removed when PR author requests a review from an assignee
+remove_labels = ["S-waiting-on-author"]
+# Those labels are added when PR author requests a review from an assignee
+add_labels = ["S-waiting-on-review"]
+```
+
+## Implementation
+
+See [`src/handlers/review_requested.rs`](https://github.com/rust-lang/triagebot/blob/HEAD/src/handlers/review_requested.rs).