README rephrased regarding NumPy installation

README.md

@@ -50,12 +50,11 @@ Installation
 ------------
 
 This software depends on [NumPy], a Python package for
-scientific computing. You must have it installed prior to installing
-gensim.
-
-It is also recommended you install a fast BLAS library before installing
-NumPy. This is optional, but using an optimized BLAS such as MKL, [ATLAS] or
-[OpenBLAS] is known to improve performance by as much as an order of
+scientific computing. Please bear in mind that building NumPy from source
+(e.g. by installing gensim on a platform which lacks NumPy .whl distribution)
+is a non-trivial task involving [linking NumPy to a BLAS library].  
+It is recommended to provide a fast one (such as MKL, [ATLAS] or
+[OpenBLAS]) which can improve performance by as much as an order of
 magnitude. On OSX, NumPy picks up its vecLib BLAS automatically,
 so you don’t need to do anything special.
 
@@ -175,6 +174,7 @@ BibTeX entry:
   [Vector Space Model]: https://en.wikipedia.org/wiki/Vector_space_model
   [unsupervised document analysis]: https://en.wikipedia.org/wiki/Latent_semantic_indexing
   [NumPy]: https://numpy.org/install/
+  [linking NumPy to a BLAS library]: https://numpy.org/devdocs/building/blas_lapack.html
   [ATLAS]: https://math-atlas.sourceforge.net/
   [OpenBLAS]: https://xianyi.github.io/OpenBLAS/
   [source tar.gz]: https://pypi.org/project/gensim/