Add documentation for triagebot new [range-diff] handler

src/SUMMARY.md

@@ -25,6 +25,7 @@
     - [Concern](./triagebot/concern.md)
     - [Documentation Updates](./triagebot/doc-updates.md)
     - [GitHub Releases](./triagebot/github-releases.md)
+    - [Git rebase (range) diff](./triagebot/range-diff.md)
     - [Issue Links](./triagebot/issue-links.md)
     - [Issue Transfer](./triagebot/transfer.md)
     - [Labeling](./triagebot/labeling.md)

src/triagebot/range-diff.md

@@ -0,0 +1,15 @@
+# Git rebase (range) diff
+
+GitHub native compare feature shows lots of unrelated changes when a force push changes the base commit of a PR. This handler post a comment after such scenario which links to triagebot `range-diff` feature.
+
+## Configuration
+
+This feature is enabled on a repository by having a empty `[range-diff]` table in `triagebot.toml`:
+
+```toml
+[range-diff]
+```
+
+## Implementation
+
+See [`src/handlers/check_commits/range_diff.rs`](https://github.com/rust-lang/triagebot/blob/HEAD/src/handlers/check_commits/force_push_range_diff.rs).