docs: link from README to post about rebase speed
For now, the best documentation we have of how fast `jj` is is the
GitHub discussion about rebase speed.
@@ -13,8 +13,9 @@ from Google.
Jujutsu is a Git-compatible
[DVCS](https://en.wikipedia.org/wiki/Distributed_version_control). It combines
-features from Git (data model, speed), Mercurial (anonymous branching, simple
-CLI [free from "the index"](docs/git-comparison.md#the-index),
+features from Git (data model,
+[speed](https://github.com/martinvonz/jj/discussions/49)), Mercurial (anonymous
+branching, simple CLI [free from "the index"](docs/git-comparison.md#the-index),
[revsets](docs/revsets.md)), and Pijul/Darcs
([first-class conflicts](docs/conflicts.md)), with features not found in either
of them ([working-copy-as-a-commit](docs/working-copy.md),
