Update README with env. variable.

README.md

@@ -68,8 +68,15 @@ Install
 You'll need : 
 * python (tested for v >= 3.5);
 * pip (optional).
+* fortran compiler (optional)
 Note that on ubuntu, you will need to use `pip3` instead of `pip` and `python3` instead of `python`. Please see the steps given in the continous integration script [workflows](.github/workflows/ci-ubuntu.yml).
 
+
+By default, the fortan evaluation of multivariate polynomial is desactivated. To enable it, set the environnement variable: `EASTEREIG_USE_FPOLY=True`. On ubuntu like system, run
+```console
+export EASTEREIG_USE_FPOLY=True
+```
+
 ### Using pip (preferred)
 Consider using `pip` over custom script (rationale [here](https://pip.pypa.io/en/stable/reference/pip_install/)).