build/pkgs/sagelib/src/README.rst: Add some text

build/pkgs/sagelib/src/README.rst

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  Sage: Open Source Mathematics Software: Standard Python Library
 =================================================================
 
-https://www.sagemath.org
+About SageMath
+--------------
+
+   "Creating a Viable Open Source Alternative to
+    Magma, Maple, Mathematica, and MATLAB"
+
+   Copyright (C) 2005-2020 The Sage Development Team
+
+   https://www.sagemath.org
+
+SageMath fully supports all major Linux distributions, recent versions of macOS, and Windows (using Cygwin or Windows Subsystem for Linux).
+
+The traditional and recommended way to install SageMath is from source via Sage-the-distribution (https://www.sagemath.org/download-source.html).  Sage-the-distribution first builds a large number of open source packages from source (unless it finds suitable versions installed in the system) and then installs the Sage Library (sagelib, implemented in Python and Cython).
+
+
+About this experimental pip-installable source distribution
+-----------------------------------------------------------
+
+This pip-installable source distribution `sagemath-standard` is an experimental distribution of the Sage Library.  Use at your own risk.
+
+Building `sagemath-standard` has a large number of system packages as prerequisites. See https://doc.sagemath.org/html/en/installation/source.html#linux-recommended-installation
+for partial lists for various systems.
+
+A modularization effort is in progress with the goal of making it possible to install parts of the Sage Library with fewer prerequisites. https://trac.sagemath.org/ticket/29705