Add note about PYTHONNOUSERSITE to README (#1806)

README.rst

@@ -57,11 +57,19 @@ environment (this is optional).
 The following command will set up a conda virtual environment, add the
 necessary package channels, and install ctapipe specified version and its dependencies::
 
-  CTAPIPE_VER=0.11.0
-  wget https://raw.githubusercontent.com/cta-observatory/ctapipe/v$CTAPIPE_VER/environment.yml
-  conda env create -n cta -f environment.yml
-  conda activate cta
-  conda install -c conda-forge ctapipe=$CTAPIPE_VER
+```
+CTAPIPE_VER=0.11.0
+wget https://raw.githubusercontent.com/cta-observatory/ctapipe/v$CTAPIPE_VER/environment.yml
+conda env create -n cta -f environment.yml
+conda activate cta
+conda install -c conda-forge ctapipe=$CTAPIPE_VER
+```
+
+**Note**: To prevent packages you installed with `pip install --user` from taking precedence over the conda environment, run:
+
+```
+conda env config vars set PYTHONNOUSERSITE=1 -n cta
+```
 
 **Note**: this environment contains many useful packages that are not strictly requirements of ctapipe.
 To get only ctapipe and its direct dependencies, just do ``conda install -c conda-forge ctapipe[=<version>]`` in an environment