Add project logo (#40)

Author: cval26

README.md

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-# FFTPACK
+![FFTPACK](media/logo-text-background.png)
 
+# FFTPACK
 
-A package of fortran subprograms for the fast fourier transform of periodic and other symmetric sequences.
+A package of Fortran subprograms for the fast Fourier transform of periodic and other symmetric sequences.
 
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 [![Actions Status](https://github.com/fortran-lang/fftpack/workflows/doc-deployment/badge.svg)](https://github.com/fortran-lang/fftpack/actions)

media/LICENSE_LOGO.md

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+The logos, tagline, and header designs in this folder are the creations of @1984logo (GitHub, Twitter). These visual assets are made available under the Creative Commons Attribution 4.0 International (CC BY 4.0) license (https://creativecommons.org/licenses/by/4.0/).
+
+License: Creative Commons Attribution 4.0 International (CC BY 4.0)
+
+Under this license, you are free to:
+
+    Share: Copy and redistribute the visual assets in any medium or format.
+    Adapt: Remix, transform, and build upon the visuals for any purpose, even commercially.
+
+Remember to provide appropriate credit to @1984logo (GitHub, Twitter) as the original creator when using these assets.