pythonPackages.{numpy,scipy,numexpr}: support MKL as BLAS

This adds support building with MKL.

doc/languages-frameworks/python.section.md

@@ -1079,8 +1079,7 @@ To modify only a Python package set instead of a whole Python derivation, use th
 Use the following overlay template:
 
 ```nix
-self: super:
-{
+self: super: {
   python = super.python.override {
     packageOverrides = python-self: python-super: {
       zerobin = python-super.zerobin.overrideAttrs (oldAttrs: {
@@ -1095,6 +1094,25 @@ self: super:
 }
 ```
 
+### How to use Intel's MKL with numpy and scipy?
+
+A `site.cfg` is created that configures BLAS based on the `blas` parameter
+of the `numpy` derivation. By passing in `mkl`, `numpy` and packages depending
+on `numpy` will be built with `mkl`.
+
+The following is an overlay that configures `numpy` to use `mkl`:
+```nix
+self: super: {
+  python36 = super.python36.override {
+    packageOverrides = python-self: python-super: {
+      numpy = python-super.numpy.override {
+        blas = super.pkgs.mkl;
+      };
+    };
+  };
+}
+```
+
 ## Contributing
 
 ### Contributing guidelines