add deprecation notice

README.md

+NOTICE
+======
+
+Active development of pyHealpix has stopped. The package will receive bug fixes
+if necessary, but otherwise the code has been integrated into the ducc0 package
+(https://gitlab.mpcdf.mpg.de/mtr/ducc), and further development is taking place
+there.
+
+Please prefer ducc0 over pyHealpix if you are starting a new project!
+
+
 pyHealpix - minimalistic Python interface for Healpix C++
 =========================================================