doc(README.md): clarify documentation structure

doc/README.md

@@ -0,0 +1,17 @@
+Fortran-lang/fftpack Documentation
+==================================
+
+Please see the [GitHub Pages site] for the Fortran-lang/fftpack documentation.
+To generate a local copy of the corresponding HTML files, 
+
+1. Set your present working directory to the Fortran-lang/fftpack project root directory,
+2. Execute `ford API-doc-FORD-file.md`, and 
+3. View the result by opening the newly generated `API-doc/` subdirectory in a web browser.
+
+This subdirectory contains 
+1. Documentation files for previous incarnations of fftpack in the [legacy](./legacy) sudirectory and
+2. Files supporting the [API-doc-.md](../API-doc-FORD-file.md) project file that [FORD] uses to generate the GitHub Pages site.
+
+
+[FORD]: https://github.com/Fortran-FOSS-Programmers/ford
+[GitHub Pages site]: https://fortran-lang.github.io/fftpack/