REL: Specify numpy needs to be pip-installed first

I guess this temporarily closes #168. Ideally we'd do something
fancy like DEICODE does, but we can change that in the future if
doable.

Author: fedarko

README.md

@@ -46,6 +46,7 @@ online [here](https://fedarko.github.io/qurro/demos/red_sea/index.html).
 You can install Qurro using [pip](https://pip.pypa.io/en/stable/):
 
 ```
+pip install numpy
 pip install qurro
 ```