add install comman

Author: yushangdi

README.md

@@ -112,6 +112,12 @@ pip install --pre torchdata --index-url https://download.pytorch.org/whl/nightly
 
 > **Note:** The nightly build of `torchdata` is required when using a PyTorch nightly. Install it from the nightly index as shown above.
 
+To install `torchtitan` as a package locally (e.g. to import it from other projects) without pulling in its dependencies, run:
+
+```bash
+pip install -e . --no-deps
+```
+
 ### Nightly builds
 
 This method requires the nightly build of PyTorch. You can replace `cu130` with another version of cuda or an AMD GPU (e.g. `rocm6.3`).