Added Windows build instructions and icon.

windows_build.txt

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+Below are notes for creating a Windows executable.
+
+An executable can only be made on a Windows host because the PyInstaller tool (https://www.pyinstaller.org/) does not support cross-compilation.
+
+On a Windows machine, do the following:
+
+1.) Install Python v3.7.x from https://www.python.org/.  (As of this writing v3.8.0 isn't supported.)  To make life easier, check the option to add Python to the PATH environment variable.
+
+2.) Using pip, install pyinstaller and colorama:
+
+    pip install pyinstaller colorama
+
+3.) Create the executable with:
+
+    pyinstaller -F --icon windows_icon.ico ssh-audit.py
+
+4.) The 'dist' folder will have the resulting ssh-audit.exe.