Documenting mercurial support

manual/src/getting_started.md

@@ -75,3 +75,27 @@ You can then run `git difftool` to see current changes with difftastic.
 ```
 $ git difftool
 ```
+
+## Mercurial: External Diffs
+
+Mercurial [supports external diff
+tools](https://www.mercurial-scm.org/wiki/ExtdiffExtension) with the
+Extdiff extension. Enable it by adding an entry to `extensions` in
+your `.hgrc`.
+
+```
+[extensions]
+extdiff =
+```
+
+You can then run `hg extdiff -p difftastic` (assumes difftastic is on
+your `$PATH`). **This currently only supports diffs where a single file
+is modified.**
+
+You can also define an alias to run difftastic with hg. Add the
+following to your `.hgrc` to run difftastic with `hg dft`.
+
+```
+[extdiff]
+cmd.dft = difftastic
+```