Add SLEEF to README. I was going to add it to the CI, but it looks as if Ubuntu's libsleef-dev doesn't include sleefdft.

.build.yml

@@ -4,6 +4,7 @@ packages:
   - libsamplerate0-dev
   - libsndfile1-dev
   - libfftw3-dev
+#  - libsleef-dev
   - ladspa-sdk
   - lv2-dev
   - vamp-plugin-sdk
@@ -19,6 +20,7 @@ tasks:
       meson build_speex -Dresampler=speex
       meson build_libsamplerate -Dresampler=libsamplerate
       meson build_fftw -Dfft=fftw
+#      meson build_sleef -Dfft=sleef
       meson build_kissfft -Dfft=kissfft
   - build: |
       cd rubberband
@@ -33,6 +35,8 @@ tasks:
       build_libsamplerate/rubberband -V
       ninja -C build_fftw
       meson test -C build_fftw
+#      ninja -C build_sleef
+#      meson test -C build_sleef
       build_fftw/rubberband -V
       ninja -C build_kissfft
       meson test -C build_kissfft