docs: recommend safer pip install

Co-authored-by: jaimergp <jaimergp@users.noreply.github.com>

README.md

@@ -124,7 +124,7 @@ Then activate the environment, check that it works and install the dependencies:
 $ conda activate conda-smithy
 $ which python
 # ☝️ should now point to an environment-specific python
-$ python -m pip install .
+$ python -m pip install -e . --no-deps
 ```
 
 You might want to switch Python versions, for example if you wish to exactly replicate a CI issue locally. To do that: