docs(fsmonitor): add filesystem monitor/Watchman documentation

CHANGELOG.md

@@ -121,6 +121,9 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 
 * `jj branch list` can now be filtered by revset.
 
+* Initial support for the Watchman filesystem monitor. Set
+  `core.fsmonitor = "watchman"` in your repo to enable.
+
 ### Fixed bugs
 
 * Modify/delete conflicts now include context lines

docs/config.md

@@ -448,6 +448,28 @@ branches. Since the local branch isn't created, the remote branch will be
 deleted if you push the branch with `jj git push --branch` or `jj git push
 --all`.
 
+## Filesystem monitor
+
+In large repositories, it may be beneficial to use a "filesystem monitor" to
+track changes to the working copy. This allows `jj` to take working copy
+snapshots without having to rescan the entire working copy.
+
+### Watchman
+
+The [Watchman filesystem monitor](https://facebook.github.io/watchman/) is
+currently only enabled if you compile jj with the `watchman` feature, such as
+with the following invocation:
+
+```shell
+cargo install --git https://github.com/martinvonz/jj.git --locked --bin jj jujutsu --features watchman
+```
+
+To configure the Watchman filesystem monitor, set
+`core.fsmonitor = "watchman"`. Ensure that you have [installed the Watchman
+executable on your system](https://facebook.github.io/watchman/docs/install)).
+
+Debugging commands are available under `jj debug watchman`.
+
 # Alternative ways to specify configuration settings
 
 Instead of `~/.jjconfig.toml`, the config settings can be located under