chore: add INSTRUCTIONS.txt file so package can be properly released

Author: zmx27

INSTRUCTIONS.txt

@@ -0,0 +1,19 @@
+xPDFsuite, a software for PDF transformation and visualization.
+GUI for diffpy.srxplanar
+
+------------------------------------------------------------------------
+1. License Notice
+
+Use of this software is permitted only under a separate, written
+Use License granted by Columbia University. If you are unsure about
+your license status, contact Prof. Simon Billinge at [email protected]
+before installing.
+
+Use without a valid license is prohibited.
+------------------------------------------------------------------------
+2. Installation
+
+This package is not designed to be installed standalone and is usually
+installed as part of the xpdfsuite package. Please refer to the
+INSTRUCTIONS.txt file and any other installation directions in the
+diffpy.xpdfsuite software.