Merge pull request #67 from Tieqiong/readme

README requirement

Author: sbillinge

.github/workflows/pre-commit.yml

@@ -63,6 +63,28 @@ by citing the following paper in your publication:
    <http://dx.doi.org/10.1107/S2053273315014473>`__,
    *Acta Crystallogr. A* **71**, 562-568 (2015).
 
+Requirements
+------------------------------------------------------------------------
+
+The diffpy.structure package requires Python 3.7 or later or 2.7 and
+the following software:
+
+* ``setuptools`` - software distribution tools for Python
+* ``NumPy`` - numerical mathematics and fast array operations for Python
+
+We recommend to use `Anaconda Python <https://www.anaconda.com/download>`_
+as it allows to install all software dependencies together with
+diffpy.structure.  For other Python distributions it is necessary to
+install the required software separately.  As an example on Ubuntu
+Linux the required software can be installed with ::
+
+   sudo aptitude install python3-setuptools python3-numpy
+
+diffpy.structure also uses the
+`PyCifRW <https://bitbucket.org/jamesrhester/pycifrw>`_
+library, which is automatically deployed during the
+installation process.
+
 Installation
 ------------