Document that uv is safe to run concurrently

README.md

@@ -261,8 +261,18 @@ The specifics of uv's caching semantics vary based on the nature of the dependen
 - **For Git dependencies**, uv caches based on the fully-resolved Git commit hash. As such,
   `uv pip compile` will pin Git dependencies to a specific commit hash when writing the resolved
   dependency set.
-- **For local dependencies**, uv caches based on the last-modified time of the `setup.py` or
-  `pyproject.toml` file.
+- **For local dependencies**, uv caches based on the last-modified time of the source archive (i.e.,
+  the local `.whl` or `.tar.gz` file). For directories, uv caches based on the last-modified time of
+  the `pyproject.toml`, `setup.py`, or `setup.cfg` file.
+
+It's safe to run multiple `uv` commands concurrently, even against the same virtual environment.
+uv's cache is designed to be thread-safe and append-only, and thus robust to multiple concurrent
+readers and writers. uv applies a file-based lock to the target virtual environment when installing,
+to avoid concurrent modifications across processes.
+
+Note that it's _not_ safe to modify the uv cache directly (e.g., `uv cache clean`) while other `uv`
+commands are running, and _never_ safe to modify the cache directly (e.g., by removing a file or
+directory).
 
 If you're running into caching issues, uv includes a few escape hatches: