note about pep517

README.md

@@ -48,6 +48,10 @@ pip install hpfrec
 ```
 (Alternatively, can also pass argument `enable-omp` to the setup.py file: `python setup.py install enable-omp`)
 
+**Note2:** the setup script uses a PEP517 environment, which means it will create an isolated virtual environment, install its build dependencies there, compile, and then copy to the actual environment. This can causes issues - for example, if one has NumPy<1.20 and the build environment installs NumPy>=1.20, there will be a binary incompatibility which will make the package fail to import. To avoid PEP517, install with:
+```
+pip install --no-use-pep517 hpfrec
+```
 
 ## Sample usage