Document usage with jj

manual/src/SUMMARY.md

@@ -7,6 +7,7 @@
   - [Git](./git.md)
   - [Mercurial](./mercurial.md)
   - [Fossil](./fossil.md)
+  - [Jujutsu](./jj.md)
 - [Languages Supported](./languages_supported.md)
   - [Language Detection](./language_detection.md)
 - [Internals: Parsing](./parsing.md)

manual/src/jj.md

@@ -0,0 +1,15 @@
+# Jujutsu
+
+Jujutsu supports [external diff
+commands](https://jj-vcs.github.io/jj/v0.26.0/config/#generating-diffs-by-external-command).
+
+To use difftastic for diffing in jujutsu, add the following to your
+[user configuration
+file](https://jj-vcs.github.io/jj/v0.26.0/config/#user-config-file).
+
+```toml
+[ui]
+diff.tool = ["difft", "--color=always", "$left", "$right"]
+```
+
+